Support DeepSeek and xAI Grok as first-class AI providers

[tutao0123]

Summary

Add first-class AI provider support for DeepSeek and xAI Grok.

Cloudflare OS currently supports a fixed set of providers and allows custom model IDs under those providers, but DeepSeek and xAI/Grok are not represented as providers in the UI or in the provider type/routing layer. Users can sometimes work around this through the generic OpenAI-compatible path, but that does not provide a clear provider-specific setup or guarantee compatibility with the request format used by the agent loop.

Why

Both services expose OpenAI-compatible APIs:

• DeepSeek: https://api.deepseek.com
• xAI: https://api.x.ai/v1

References:

• https://api-docs.deepseek.com/zh-cn/guides/function_calling/
• https://x.ai/

Adding them as first-class providers would make setup discoverable and allow provider-specific model metadata and compatibility handling.

Proposed scope

• Add deepseek and xai to AiModelProvider.
• Add provider labels, default API URLs, API-token hints, and suggested/current model IDs.
• Route both providers through the appropriate pi-ai streaming adapter.
• Preserve support for custom model IDs.
• Verify streaming responses, tool calls, reasoning output, image capability metadata, and multi-turn agent runs.
• Add unit/integration coverage with mocked provider endpoints.
• Keep the existing generic OpenAI-compatible configuration working unchanged.

If maintaining two separate provider types is undesirable, a generic OpenAI-compatible provider with DeepSeek and xAI presets would also solve the setup/discoverability problem.

Acceptance criteria

• A user can select DeepSeek or xAI/Grok from AI Providers without pretending it is OpenAI.
• The default endpoint and credential instructions are shown automatically, while allowing an API URL override where appropriate.
• At least one current DeepSeek model and one current Grok model can complete a normal chat and an agent tool-call loop.
• Model capability metadata prevents unsupported attachments or features from being sent.
• Existing OpenAI, Anthropic, Google, Cloudflare Workers AI, and Ollama configurations are not regressed.