feat(veo): add Veo 3.1 model support

[CharlesWong]

Adds support for Veo 3.1 models in the Vertex AI veo provider.

Changes

• Added veo-3.1-generate-preview and veo-3.1-fast-generate-preview to VertexAIVideoModels with pricing
• Updated VertexAIVideoModel type union to include both new Veo 3.1 models
• Added Veo 3.1 models to the VEO_MODELS list in VertexAIProvider (renamed from VEO3_MODELS for future-proofing; likewise isVeoModel replaces isVeo3Model)
• Updated VideoModelOption type in the Next.js template to include Veo 3.1 variants
• Updated request validation to accept Veo 3.1 model IDs
• Updated handleGeminiGenerate function signature to accept Veo 3.1 models
• Added Veo 3.1 models to the video generator component model list
• Set veo-3.1-fast-generate-preview as the default model in the Next.js video template

Pricing

Veo 3.1 pricing matches Veo 3.0 (per Vertex AI pricing docs):

• veo-3.1-generate-preview: $0.40/second with audio, $0.20/second without audio
• veo-3.1-fast-generate-preview: $0.15/second with audio, $0.10/second without audio

Closes #595

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[CharlesWong]

Updated: Added Gemini SDK support and improved model ordering

Changes in this update:

• Added Veo 3.1 models to packages/sdk/ts/src/supported-models/video/gemini.ts — previously we only covered vertex-ai.ts, now Gemini provider also has veo-3.1-generate-preview and veo-3.1-fast-generate-preview with correct pricing
• Refactored vertex.ts to use VideoModelOption type import instead of inline union type (cleaner, DRY)
• Reordered model lists — Veo 3.1 models now appear first in types.ts, validation.ts, video-generator.tsx, and gemini.ts since they are the latest models
• Default model changed to veo-3.1-fast-generate-preview
• Rebased onto latest upstream/master