fix: defer CLI path lookup to async context to prevent blocking calls

- Move shutil.which() call from __init__ to connect() method
- Use asyncio.to_thread() to run _find_cli() in background thread
- Prevents blockbuster library from detecting os.access as blocking call
- Maintains backward compatibility and minimal code changes

Resolves issue where SDK would fail in async environments with
blocking call detection libraries like blockbuster.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/claude_agent_sdk/_internal/transport/subprocess_cli.py

@@ -1,5 +1,6 @@
 """Subprocess transport implementation using Claude Code CLI."""
 
+import asyncio
 import json
 import logging
 import os
@@ -48,9 +49,7 @@ class SubprocessCLITransport(Transport):
         self._prompt = prompt
         self._is_streaming = not isinstance(prompt, str)
         self._options = options
-        self._cli_path = (
-            str(options.cli_path) if options.cli_path is not None else self._find_cli()
-        )
+        self._cli_path = str(options.cli_path) if options.cli_path is not None else None
         self._cwd = str(options.cwd) if options.cwd else None
         self._process: Process | None = None
         self._stdout_stream: TextReceiveStream | None = None
@@ -266,6 +265,10 @@ class SubprocessCLITransport(Transport):
         if self._process:
             return
 
+        # Defer CLI path lookup to async context to avoid blocking calls
+        if self._cli_path is None:
+            self._cli_path = await asyncio.to_thread(self._find_cli)
+
         if not os.environ.get("CLAUDE_AGENT_SDK_SKIP_VERSION_CHECK"):
             await self._check_claude_version()