Address anyio.BrokenResourceError issue from #139 (#149)

## Summary

This PR addresses the `anyio.BrokenResourceError` issue from #139 and
aligns the Python SDK's error handling with the TypeScript SDK
implementation.

## Changes

### Fix stream closure issue in Query
- Removed the `async with` context manager from
`Query.receive_messages()` that was closing the stream after first use
- The stream now remains open for the entire session, allowing multiple
queries in streaming mode
- This fixes the `BrokenResourceError` that occurred during multi-turn
conversations

### Align subprocess transport with TypeScript SDK
Following the TypeScript `ProcessTransport` implementation pattern:

- **Added `_exit_error` tracking**: Captures and preserves process-level
errors for better error propagation
- **Enhanced `write()` method checks**: 
  - Validates transport readiness before writing
  - Checks if process is still alive (exit code)
  - Checks for stored exit errors before attempting writes
  - Marks transport as not ready on write failures
- **Improved error handling in `connect()`**: Stores errors as
`_exit_error` for later reference
- **Simplified `is_ready()` method**: Now just returns the `_ready`
flag, matching TypeScript's simpler approach

### Other improvements
- Added asyncio pytest plugin configuration (`-p asyncio` in
pyproject.toml)
- Added clarifying comment about TextReceiveStream line handling

## Testing

The multi-turn conversation example now works correctly:
```bash
python examples/streaming_mode.py multi_turn_conversation
```

## Related Issues

Fixes #139

pyproject.toml

@@ -61,6 +61,7 @@ testpaths = ["tests"]
 pythonpath = ["src"]
 addopts = [
     "--import-mode=importlib",
+    "-p", "asyncio",
 ]
 
 [tool.pytest-asyncio]

src/claude_code_sdk/_internal/query.py

@@ -456,15 +456,14 @@ class Query:
 
     async def receive_messages(self) -> AsyncIterator[dict[str, Any]]:
         """Receive SDK messages (not control messages)."""
-        async with self._message_receive:
-            async for message in self._message_receive:
-                # Check for special messages
-                if message.get("type") == "end":
-                    break
-                elif message.get("type") == "error":
-                    raise Exception(message.get("error", "Unknown error"))
+        async for message in self._message_receive:
+            # Check for special messages
+            if message.get("type") == "end":
+                break
+            elif message.get("type") == "error":
+                raise Exception(message.get("error", "Unknown error"))
 
-                yield message
+            yield message
 
     async def close(self) -> None:
         """Close the query and transport."""

src/claude_code_sdk/_internal/transport/subprocess_cli.py

@@ -46,6 +46,7 @@ class SubprocessCLITransport(Transport):
         self._stdin_stream: TextSendStream | None = None
         self._stderr_file: Any = None  # tempfile.NamedTemporaryFile
         self._ready = False
+        self._exit_error: Exception | None = None  # Track process exit errors
 
     def _find_cli(self) -> str:
         """Find Claude Code CLI binary."""
@@ -213,12 +214,18 @@ class SubprocessCLITransport(Transport):
         except FileNotFoundError as e:
             # Check if the error comes from the working directory or the CLI
             if self._cwd and not Path(self._cwd).exists():
-                raise CLIConnectionError(
+                error = CLIConnectionError(
                     f"Working directory does not exist: {self._cwd}"
-                ) from e
-            raise CLINotFoundError(f"Claude Code not found at: {self._cli_path}") from e
+                )
+                self._exit_error = error
+                raise error from e
+            error = CLINotFoundError(f"Claude Code not found at: {self._cli_path}")
+            self._exit_error = error
+            raise error from e
         except Exception as e:
-            raise CLIConnectionError(f"Failed to start Claude Code: {e}") from e
+            error = CLIConnectionError(f"Failed to start Claude Code: {e}")
+            self._exit_error = error
+            raise error from e
 
     async def close(self) -> None:
         """Close the transport and clean up resources."""
@@ -259,13 +266,34 @@ class SubprocessCLITransport(Transport):
         self._stdout_stream = None
         self._stderr_stream = None
         self._stdin_stream = None
+        self._exit_error = None
 
     async def write(self, data: str) -> None:
         """Write raw data to the transport."""
-        if not self._stdin_stream:
-            raise CLIConnectionError("Cannot write: stdin not available")
+        # Check if ready (like TypeScript)
+        if not self._ready or not self._stdin_stream:
+            raise CLIConnectionError("ProcessTransport is not ready for writing")
 
-        await self._stdin_stream.send(data)
+        # Check if process is still alive (like TypeScript)
+        if self._process and self._process.returncode is not None:
+            raise CLIConnectionError(
+                f"Cannot write to terminated process (exit code: {self._process.returncode})"
+            )
+
+        # Check for exit errors (like TypeScript)
+        if self._exit_error:
+            raise CLIConnectionError(
+                f"Cannot write to process that exited with error: {self._exit_error}"
+            ) from self._exit_error
+
+        try:
+            await self._stdin_stream.send(data)
+        except Exception as e:
+            self._ready = False  # Mark as not ready (like TypeScript)
+            self._exit_error = CLIConnectionError(
+                f"Failed to write to process stdin: {e}"
+            )
+            raise self._exit_error from e
 
     async def end_input(self) -> None:
         """End the input stream (close stdin)."""
@@ -273,9 +301,6 @@ class SubprocessCLITransport(Transport):
             with suppress(Exception):
                 await self._stdin_stream.aclose()
             self._stdin_stream = None
-        if self._process and self._process.stdin:
-            with suppress(Exception):
-                await self._process.stdin.aclose()
 
     def read_messages(self) -> AsyncIterator[dict[str, Any]]:
         """Read and parse messages from the transport."""
@@ -295,6 +320,9 @@ class SubprocessCLITransport(Transport):
                 if not line_str:
                     continue
 
+                # Accumulate partial JSON until we can parse it
+                # Note: TextReceiveStream can truncate long lines, so we need to buffer
+                # and speculatively parse until we get a complete JSON object
                 json_lines = line_str.split("\n")
 
                 for json_line in json_lines:
@@ -361,21 +389,16 @@ class SubprocessCLITransport(Transport):
 
         # Use exit code for error detection, not string matching
         if returncode is not None and returncode != 0:
-            raise ProcessError(
+            self._exit_error = ProcessError(
                 f"Command failed with exit code {returncode}",
                 exit_code=returncode,
                 stderr=stderr_output,
             )
+            raise self._exit_error
         elif stderr_output:
             # Log stderr for debugging but don't fail on non-zero exit
             logger.debug(f"Process stderr: {stderr_output}")
 
     def is_ready(self) -> bool:
         """Check if transport is ready for communication."""
-        return (
-            self._ready
-            and self._process is not None
-            and self._process.returncode is None
-        )
-
-    # Remove interrupt and control request methods - these now belong in Query class
+        return self._ready