Support DeepSeek and xAI Grok as first-class AI providers

## 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.

Start by locating the AiModelProvider type, AI Providers UI, provider routing layer, and the pi-ai streaming adapter path. Review the existing generic OpenAI-compatible configuration and add mocked provider endpoint coverage for streaming, tool calls, reasoning, image metadata, and multi-turn agent runs. Done means DeepSeek and xAI/Grok are selectable with defaults and overrides while existing providers remain unchanged.

Written by the indexing model from the issue text.