Fix Windows subprocess stdin buffering issue causing ClaudeSDKClient hangs

[ashwin-ant]

Problem

ClaudeSDKClient initialization would hang indefinitely on Windows, timing out after 60 seconds. The SDK successfully spawned the Claude CLI subprocess but control requests sent via stdin never reached the subprocess due to Windows subprocess stdin buffering behavior with Python's asyncio.

Root Cause

On Windows, when using asyncio subprocess streams, data written to stdin can remain buffered and not immediately sent to the child process. The CLI subprocess waits for the initialization request that's stuck in Python's buffer, causing the 60-second timeout.

Solution

1. Added flush_stdin() method to Transport base class (non-abstract, default no-op)
2. Implemented Windows-specific flush in SubprocessCLITransport that calls drain() on the asyncio StreamWriter when available
3. Call flush_stdin() after all control protocol writes in Query class:
   • After sending control requests (_send_control_request)
   • After responding to incoming requests (_handle_control_request)

Tests Added

• test_flush_stdin_on_windows: Verifies drain() called on Windows
• test_flush_stdin_on_non_windows: Verifies no-op on other platforms
• test_flush_stdin_without_process: Tests graceful handling of missing process
• test_flush_stdin_fallback_to_inner_stream: Tests wrapped stream fallback
• test_flush_stdin_called_after_control_requests: Integration test for outgoing requests
• test_flush_stdin_called_after_control_responses: Integration test for incoming requests

All tests pass on macOS, and the fix is platform-specific to Windows only.

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