Add stt utils tests

[Divyansh1552005]

• What kind of change does this PR introduce?
  Tests

• Why was this change needed?
  Several internal Speech-to-Text (STT) utility helper functions were not directly covered by unit tests. These functions are critical for text normalization, prefix matching, overlap detection, and commit logic used in live transcription stabilization.

Adding dedicated tests improves reliability, prevents regressions, and increases confidence in core STT processing logic.

• Other information

What changed:

• Added unit tests for:
  • _normalize_word
  • _split_words
  • _common_prefix_length
  • _find_suffix_prefix_overlap
  • _calculate_commit_count
• Covered edge cases such as:
  • Empty and whitespace-only inputs
  • Case-insensitive matching
  • Special character normalization
  • Prefix matching correctness
  • Overlap thresholds
  • Commit behavior under normal and silence conditions

Tests / validation:

• All tests pass successfully
• Coverage maintained at a high level

Test results:
TOTAL 16825 560 97%
Coverage HTML written to dir htmlcov
Coverage XML written to file coverage.xml
======================== 4892 passed, 2 skipped, 2 warnings in 180.86s (0:03:00) =========================

Notes for reviewers:

• This PR focuses only on adding missing unit test coverage for internal helper functions
• No changes were made to existing logic or behavior
• Improves test depth without affecting existing functionality

[vercel]

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