feat(gemini): add Google Veo video adapter on the typed-duration contract

[tombeckenham]

Build the Google Gemini Veo video adapter on top of the per-model typed-duration contract introduced in #534.

Why now

#534 ships the typed-duration VideoAdapter contract (availableDurations + snapDuration + per-model duration union). Building the Veo adapter directly on the new contract means it never ships with the old duration?: number API and never breaks.

Current state

• packages/typescript/ai-gemini/src/adapters/ has no video.ts.
• packages/typescript/ai-gemini/src/model-meta.ts contains commented-out Veo model entries at lines 827–940 (VEO_3_1_PREVIEW, VEO_3_1_FAST_PREVIEW, VEO_3, VEO_3_FAST, VEO_2).
• testing/e2e/src/lib/feature-support.ts does not list 'gemini' for 'video-gen'.

Scope

1. Uncomment + finalize the Veo entries in model-meta.ts and add a GeminiVideoModel union.
2. Add packages/typescript/ai-gemini/src/video/video-provider-options.ts with GeminiVideoModelDurationByName, GeminiVideoModelSizeByName, and a GEMINI_VIDEO_DURATIONS map sourced from @tanstack/ai-schemas (the gemini OpenAPI spec is already in feat(schemas): @tanstack/ai-schemas with nightly OpenAPI sync (closes #619) #622's pipeline).
3. Build packages/typescript/ai-gemini/src/adapters/video.ts extending BaseVideoAdapter with the fifth generic and implementing the three core methods (createVideoJob, getVideoStatus, getVideoUrl) plus overriding availableDurations / snapDuration.
4. Export geminiVideo / createGeminiVideo factories from packages/typescript/ai-gemini/src/index.ts.
5. Add 'gemini' to the video-gen feature-support set and record an E2E fixture.

Per-model duration shapes (from Veo API docs)

• veo-2.0-generate-001: integer seconds 5 | 6 | 7 | 8
• veo-3.0-generate-001, veo-3.0-fast-generate-001: 4 | 8
• veo-3.1-generate-preview, veo-3.1-fast-generate-preview: 4 | 8

(Confirm against @tanstack/ai-schemas Gemini endpoint Zod map before locking the constants — schema should be the source of truth.)

API surface

Veo uses Google's :predictLongRunning endpoint. Map:

• createVideoJob → POST {model}:predictLongRunning with parameters: { durationSeconds }.
• getVideoStatus → GET /operations/{name} and map done/error.
• getVideoUrl → extract response.generatedSamples[0].video.uri (GCS signed URL).

Acceptance

• geminiVideo('veo-3.0-generate-001') has duration?: 4 | 8 typed at compile time.
• adapter.availableDurations() returns { kind: 'discrete', values: [4, 8] }.
• adapter.snapDuration(7) returns 8.
• E2E test exercises one full job lifecycle against the gemini aimock fixture.