feat(google-vertex): add moonshotai/kimi-k2-thinking-maas model

[dpark01]

Adds Kimi K2 Thinking (Moonshot AI) as a Vertex AI MaaS model entry under the google-vertex provider.

Kimi K2 Thinking is available on Vertex AI Model Garden at the global endpoint as moonshotai/kimi-k2-thinking-maas. Without this entry, OpenCode does not recognize the model as a MaaS model and incorrectly routes requests to the Gemini API endpoint, resulting in a 404. Adding the TOML entry here enables the correct MaaS routing path.

The model has been verified working via direct API call against the Vertex MaaS endpoint.

References:

• Vertex AI model card: https://console.cloud.google.com/vertex-ai/publishers/moonshotai/model-garden/kimi-k2-thinking-maas
• Vertex AI MaaS docs: https://cloud.google.com/vertex-ai/generative-ai/docs/maas/kimi