fix(websocket): finalize file input on end control

[LauraGPT]

Summary

• finalize buffered offline, online, and 2pass audio when file clients send the end control message
• add an explicit end-of-input acknowledgement so file clients wait for final results instead of fixed sleeps
• propagate inference, delivery, and invalid-configuration failures to the client while retaining legacy is_speaking: false support
• clean up failed websocket sessions and document the completion protocol

Validation

• pytest -q tests/test_websocket_file_finalization.py (28 passed)
• expanded websocket/server/docs regression selection (237 passed)
• python -m py_compile for server, client, and regression test
• black --check tests/test_websocket_file_finalization.py
• git diff --check

Fixes #3438

[gemini-code-assist]

Caution

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