feat: add Veo 3.1 video model support

[nath-netizen]

Summary

• add Veo 3.1 Vertex AI video model IDs to the TypeScript SDK model list
• update the Next video template model types, validation, API handler, and selector
• default the video template to veo-3.1-fast-generate-001
• keep the preview model IDs requested in the issue alongside the current GA model IDs
• add SDK coverage for the new Veo 3.1 model entries

Closes #595.

Notes

Google's current Vertex AI docs list GA Veo 3.1 IDs and mark the preview IDs for migration. This keeps the preview IDs requested in the issue while using the GA fast model as the template default.

Verification

• git diff --check
• attempted local SDK test install/run; the local JS runner hung before producing test output in this workspace, so the included regression test has not been executed here

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