Support Veo 3.1 in the veo provider

[rsproule]

Google has recently added a new video model:

• veo-3.1-generate-preview
• veo-3.1-fast-generate-preview

Echo router should support these.

Router:

• Should be very minimal changes to the router, just add support for this model string.

Template:

• Update the video template to have these new models as options (they should be the default)

[Cyber-Mitch]

/attempt #595

[buildingvibes]

/attempt #595

Added Veo 3.1 support across the router, SDK, and video template. PR: #718

Changes cover all areas mentioned in the issue:

• Router: Added veo-3.1-fast-generate-preview and veo-3.1-generate-preview to supported models with pricing
• Template: Added new models to the dropdown with Veo 3.1 Fast as the default, updated validation and handler types

[285729101]

PR here: #725

Added both veo-3.0-generate-preview and veo-3.1-generate with pricing from the Gemini API docs. Updated the SDK models, the router provider, and the video template dropdown.