fix(copilot): clear O_NONBLOCK on subprocess pipes to prevent BlockingIOError

[jrob5756]

Summary

• Adds _fix_pipe_blocking_mode() to CopilotProvider that clears O_NONBLOCK on the Copilot CLI subprocess stdin/stdout pipes after startup
• Prevents BlockingIOError when large JSON-RPC messages (e.g., prompts with many gathered articles) exceed the OS pipe buffer
• Safe because the SDK already runs pipe writes in a thread-pool executor where blocking is correct

Test plan

• Verify existing tests pass (make test)
• Test with large prompt payloads that previously triggered BlockingIOError
• Confirm normal workflow execution is unaffected

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