feat(asr): add Deepgram streaming transport

[quanru]

Summary

• add owner-only Deepgram API-key storage with validation and safe-file checks
• add a session-owned Nova-3 WebSocket transport for 16 kHz mono PCM
• stream interim snapshots, accumulate finalized segments, and close with Deepgram Finalize/CloseStream messages
• classify handshake and protocol authentication failures separately from network failures

Dependency

Depends on #45. This PR deliberately does not register Deepgram in the product UI.

Validation

• PYTHONPATH=src /Users/bytedance/personal/doubao-say/.venv/bin/python -m unittest tests.unit.test_deepgram_credentials tests.unit.test_deepgram_asr_client
• /Users/bytedance/personal/doubao-say/.venv/bin/python -m ruff check src tests packaging
• PYTHONPATH=src /Users/bytedance/personal/doubao-say/.venv/bin/python -m compileall -q src/doubao_input/deepgram tests/unit/test_deepgram_asr_client.py tests/unit/test_deepgram_credentials.py
• git diff --check

Validation gap

No live Deepgram credential was used. Protocol behavior is covered with a fake bidirectional WebSocket; Linux CI provides the full repository test matrix.

Review order

Review after #45 and before the Deepgram product-integration PR.