feat(providers): rewrite Anthropic and OpenAI providers
- Anthropic Messages API (2023-06-01) SSE streaming: text/thinking/tool_use
blocks, cache usage, stop-reason mapping, errors encoded as events
- OpenAI Responses API + Chat Completions (switchable), tool_calls delta
accumulation, include_usage, reasoning_content -> thinking
- ProviderConfig with base_url/api_key/context_window/timeout; known-model
context-window table with three-layer resolution
- background current-thread runtime bridges sync agent loop to async I/O
- mock-transport replay tests (chunks straddle SSE boundaries) + full agent
integration test
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