feat: add native Anthropic Messages API support via adapter pattern

[Program120]

Summary

• Add AnthropicAdapter — translates Anthropic Messages API to OpenAI-compatible format
• Restore PROVIDER_PRESETS base_url fallback with new anthropic preset
• Convert httpx exceptions to urllib errors for unified retry mechanism
• Support json_mode via system prompt injection for Anthropic provider
• Add httpx as optional dependency (pip install researchclaw[anthropic])
• Add detailed changelog with architecture diagrams: docs/CHANGELOG_ANTHROPIC_ADAPTER.md

Zero Impact on Existing Providers

AnthropicAdapter is only instantiated when provider == "anthropic".
For all other providers, self._anthropic remains None — code path is identical to before.

Test Plan

• Set ANTHROPIC_API_KEY and run python tests/test_anthropic.py
• Verify OpenAI/OpenRouter/DeepSeek providers still work
• Confirm no hardcoded API keys in diff

[Jiaaqiliu]

Hi @Program120, thanks for this well-structured contribution!

We've reviewed and merged this PR. The Anthropic Messages API adapter is now part of main, enabling native Claude model support.

What we did during integration:

• Resolved merge conflicts in client.py and __init__.py — our MetaClaw bridge feature was added in parallel, so we carefully merged the Anthropic adapter's if self._anthropic branching with the MetaClaw fallback logic.
• The PROVIDER_PRESETS dict and from_rc_config() factory method are both integrated cleanly.

During our post-merge code audit, we identified a few areas for future improvement:

1. stop_reason mapping: Anthropic returns "max_tokens" / "end_turn", but the downstream code expects OpenAI-style "length" / "stop". This means truncated detection is currently a no-op for Anthropic responses. A mapping dict would fix this.
2. Multi-part content: The adapter only extracts the first content block (data["content"][0]). Anthropic can return multiple text blocks — concatenating all type: "text" blocks would be more robust.
3. MetaClaw bridge interaction: When both Anthropic provider and MetaClaw bridge are enabled, the adapter receives the proxy URL instead of the Anthropic API URL. This combination should either be guarded against or handled explicitly.
4. httpx.Client per-call: Creating a new httpx.Client on each API call loses connection pooling. Reusing a single client instance in __init__ would improve performance.

None of these are blockers — the adapter works correctly for the standard use case (direct Anthropic API). Would love to see follow-up PRs addressing any of these!

Excellent adapter pattern and great documentation in the changelog. Thanks for contributing!