Add ProviderCapabilities struct to enable runtime detection of
provider-specific features, starting with native tool calling support.
This is a foundational change that enables future PRs to implement
intelligent tool calling mode selection (native vs prompt-guided).
Changes:
- Add ProviderCapabilities struct with native_tool_calling field
- Add capabilities() method to Provider trait with default impl
- Add unit tests for capabilities equality and defaults
Why:
- Current design cannot distinguish providers with native tool calling
- Needed to enable Gemini/Anthropic/OpenAI native function calling
- Fully backward compatible (all providers inherit default)
What did NOT change:
- No existing Provider methods modified
- No behavior changes for existing code
- Zero breaking changes
Testing:
- cargo test: all tests passed
- cargo fmt: pass
- cargo clippy: pass
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