Add support for custom env vars (#131)

## Key changes
- Adds env field to `ClaudeCodeOptions`, allowing custom env vars to cli
- Updates tests and examples

## Motivation
Bringing Python SDK to feature parity with TS SDK, which supports custom
env vars

## Notes
- Environment variables are merged in order: system env → user env → SDK
required vars
- This implementation seems slightly more robust than the TypeScript
version, which can exclude OS envs vars if a user passes a minimal env
object
- Some linting changes seem to have been picked up

tests/test_transport.py

@@ -1,5 +1,7 @@
 """Tests for Claude SDK transport layer."""
 
+import os
+import uuid
 from unittest.mock import AsyncMock, MagicMock, patch
 
 import anyio
@@ -299,3 +301,54 @@ class TestSubprocessCLITransport:
         assert "--mcp-config" in cmd
         mcp_idx = cmd.index("--mcp-config")
         assert cmd[mcp_idx + 1] == json_config
+
+    def test_env_vars_passed_to_subprocess(self):
+        """Test that custom environment variables are passed to the subprocess."""
+
+        async def _test():
+            test_value = f"test-{uuid.uuid4().hex[:8]}"
+            custom_env = {
+                "MY_TEST_VAR": test_value,
+            }
+
+            options = ClaudeCodeOptions(env=custom_env)
+
+            # Mock the subprocess to capture the env argument
+            with patch(
+                "anyio.open_process", new_callable=AsyncMock
+            ) as mock_open_process:
+                mock_process = MagicMock()
+                mock_process.stdout = MagicMock()
+                mock_stdin = MagicMock()
+                mock_stdin.aclose = AsyncMock()  # Add async aclose method
+                mock_process.stdin = mock_stdin
+                mock_process.returncode = None
+                mock_open_process.return_value = mock_process
+
+                transport = SubprocessCLITransport(
+                    prompt="test",
+                    options=options,
+                    cli_path="/usr/bin/claude",
+                )
+
+                await transport.connect()
+
+                # Verify open_process was called with correct env vars
+                mock_open_process.assert_called_once()
+                call_kwargs = mock_open_process.call_args.kwargs
+                assert "env" in call_kwargs
+                env_passed = call_kwargs["env"]
+
+                # Check that custom env var was passed
+                assert env_passed["MY_TEST_VAR"] == test_value
+
+                # Verify SDK identifier is present
+                assert "CLAUDE_CODE_ENTRYPOINT" in env_passed
+                assert env_passed["CLAUDE_CODE_ENTRYPOINT"] == "sdk-py"
+
+                # Verify system env vars are also included with correct values
+                if "PATH" in os.environ:
+                    assert "PATH" in env_passed
+                    assert env_passed["PATH"] == os.environ["PATH"]
+
+        anyio.run(_test)