test(e2e): add Gemini TTS coverage to provider matrix

[harshlocham]

🎯 Changes

Gemini TTS is already implemented and covered by unit tests, but it was not participating in the E2E TTS provider matrix because the provider-specific E2E wiring was incomplete.

This PR adds Gemini TTS to the existing E2E infrastructure by:

• Adding a Gemini TTS adapter factory in media-providers.ts
• Adding a Gemini generateContent audio mock in global-setup.ts
• Adding gemini to the TTS provider support matrix
• Exercising the adapter's PCM-to-WAV normalization path using raw PCM audio responses (audio/L16;codec=pcm;rate=24000)

No runtime behavior, public APIs, or package functionality are changed. This PR only extends test coverage for an existing feature.

Verification

• Gemini TTS E2E tests pass (SSE, HTTP stream, and fetcher modes)
• Full TTS provider matrix passes
• No changes outside test infrastructure

✅ Checklist

• I have followed the steps in the Contributing guide.
• I have tested this code locally with pnpm run test:pr.

🚀 Release Impact

• This change affects published code, and I have generated a changeset.
• This change is docs/CI/dev-only (no release).

Summary by CodeRabbit

• Tests
  • Expanded end-to-end test coverage to include Gemini Text-to-Speech provider with audio format validation.
  • Updated test provider support matrix to include Gemini TTS testing capabilities.
  • Implemented Gemini TTS adapter integration for test infrastructure.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 9ac1a081-6bcb-48a3-81d2-e8e8f44d1b4e

📥 Commits

Reviewing files that changed from the base of the PR and between 984ac3c and 7171f79.

📒 Files selected for processing (3)

• testing/e2e/global-setup.ts
• testing/e2e/src/lib/feature-support.ts
• testing/e2e/src/lib/media-providers.ts

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📝 Walkthrough

Walkthrough

This PR adds end-to-end test support for Gemini TTS by declaring the provider in the feature matrix, wiring the TTS adapter factory with test configuration, and implementing a mock Gemini generateContent endpoint that returns PCM-encoded audio to exercise the adapter's normalization path.

Changes

Gemini TTS End-to-End Testing

Layer / File(s)	Summary
Feature support matrix testing/e2e/src/lib/feature-support.ts	Declares gemini as a supported TTS provider in the e2e test matrix alongside openai, grok, and elevenlabs.
Gemini TTS adapter factory testing/e2e/src/lib/media-providers.ts	Reformats Gemini imports and adds a gemini entry to the createTTSAdapter factory map configured with test base URL and headers.
Gemini TTS mock endpoint and registration testing/e2e/global-setup.ts	Adds FAKE_PCM_BYTES test data, implements geminiTTSMount() to return Gemini-shaped JSON with base64-encoded PCM audio, and mounts it in globalSetup.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

🐰 A Gemini speaks now, in audio so clear,
With PCM bytes and a mock setup dear!
The TTS path flows, from test to the sound,
E2E magic—more providers found! 🎵

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and specifically describes the main change: adding Gemini TTS coverage to the E2E provider matrix, which aligns with the changeset modifications.
Description check	✅ Passed	The description covers all required template sections with substantive content: changes are explained, checklist items are marked, and release impact is properly documented as dev-only.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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