[codex] Isolate Parakeet stream language detection tests

[tianmind-studio]

Summary

• Add a lightweight langdetect / LangDetectException stub to test_parakeet_stream_session.py.
• Keep Parakeet stream-session tests runnable in lightweight Windows backend unit environments without installing the full language-detection dependency.
• Preserve the file's existing transcribe, Scipy, and torch stubs.

Root cause

stream_handler.py imports langdetect at module import time for detected-language metadata. In a lightweight Windows backend unit venv without full backend requirements installed, test_parakeet_stream_session.py could not collect even though the tested session behavior does not require the real language detector.

Before this change:

• python -m pytest backend\tests\unit\test_parakeet_stream_session.py --collect-only -q --tb=short -> ModuleNotFoundError: No module named 'langdetect'

Validation

• python -m pytest backend\tests\unit\test_parakeet_stream_session.py --collect-only -q --tb=short -> 14 tests collected
• python -m pytest backend\tests\unit\test_parakeet_stream_session.py -q --tb=short -> 14 passed
• python -m pytest backend\tests\unit\test_action_item_date_validation.py backend\tests\unit\test_parakeet_stream_session.py -q --tb=short -> 40 passed
• python -m black --check --line-length 120 --skip-string-normalization backend\tests\unit\test_parakeet_stream_session.py
• python -m py_compile backend\tests\unit\test_parakeet_stream_session.py
• git diff --check
• scripts/pre-commit