Fix Gemini provider filtering in voice utilities

[OwenMcGirr]

Summary

• Fix VoiceUtils.filterByProvider() to use the canonical UnifiedVoice["provider"] type instead of a stale hard-coded provider union.
• Add a regression test proving Gemini unified voices can be filtered with filterByProvider(voices, "gemini").

Fixes #59

Validation

• npm test -- --runTestsByPath src/__tests__/gemini.test.ts src/__tests__/voice-utils.test.ts --runInBand
• npm run build
• pre-push npm run lint

[OwenMcGirr]

Smoke verified on ix/59-gemini-provider-filter:

pm run smoke:bundle

• Browser bundle smoke OK. Bytes: 1581188. Warnings: no
• Root package browser smoke OK. Bytes: 1165636. Warnings: no

[OwenMcGirr]

Sibling smoke verified from ../js-tts-wrapper-gemini-test against the local file:../js-tts-wrapper junction:

• npm test
  • Gemini voices: 30
  • Gemini language metadata OK: 87 codes
  • CereVoice smoke test passed
• Inline provider-filter smoke:
  • VoiceUtils.filterByProvider(voices, "gemini") returned 30 voices

[OwenMcGirr]

Additional sibling smoke for Gemini gender filtering passed from ../js-tts-wrapper-gemini-test:

• client.getVoicesByGender("Male") returned 16 voices
• client.getVoicesByGender("Female") returned 14 voices
• VoiceUtils.filterByGender(VoiceUtils.filterByProvider(voices, "gemini"), "Male") returned 16 voices
• VoiceUtils.filterByGender(VoiceUtils.filterByProvider(voices, "gemini"), "Female") returned 14 voices
• Verified Puck appears in the male set and Zephyr appears in the female set

[OwenMcGirr]

Post-merge CereVoice parity check passed:

• Local repo: npm test -- --runTestsByPath src/__tests__/cerevoice.test.ts --runInBand passed, 12/12 tests.
• Sibling smoke from ../js-tts-wrapper-gemini-test with mocked male/female CereVoice voices passed:
  • VoiceUtils.filterByProvider(voices, "cerevoice") returned 2 voices
  • client.getVoicesByGender("Male") returned William
  • client.getVoicesByGender("Female") returned Heather
  • helper gender filters after provider filtering returned the same split