Add Workers AI gateway for direct API integration

[meirdick]

Summary

Adds a native WorkersAiGateway that calls the Cloudflare Workers AI API directly via the Chat Completions format, following the same trait-based composition pattern as the Groq and Mistral gateway migrations.

• Provider: WorkersAiProvider with text generation + embeddings
• Gateway: WorkersAiGateway implements EmbeddingGateway, TextGateway
• API format: Chat Completions (like Groq, Mistral) — not Responses API
• URL construction: Auto-builds from account_id (direct) or gateway config (AI Gateway)
• Reasoning models: ReasoningStart/Delta/End events for Kimi K2.5, Gemma 4, QwQ
• Session affinity: x-session-affinity header for prefix caching
• Content coercion: String content for Workers AI compatibility
• Validation: Helpful error when /compat URL used without workers-ai/ model prefix

User setup

# Direct (minimal)
CLOUDFLARE_AI_API_KEY=your-token
CLOUDFLARE_ACCOUNT_ID=abc123

# With AI Gateway (add one line)
WORKERSAI_GATEWAY=my-gateway-name

Test plan

• 39 Workers AI provider tests (BaseUrl, ErrorHandling, MessageMapping, RequestMapping, Streaming, ToolCallLoop, ToolMapping, ProviderOptions, AgentFake)
• 527 total tests pass (zero regressions)
• Verified against real Cloudflare API (direct, AI Gateway, /compat paths)
• Browser tested on 4 projects: career-forge, nex, endo-baddie, paws
• Streaming with reasoning events confirmed working (Kimi K2.5)
• Embeddings confirmed working
• Tool calling with multi-step loops confirmed working

🤖 Generated with Claude Code