story-kit: create 123_story_test_coverage_llm_providers_anthropic_rs

.story_kit/work/1_upcoming/123_story_test_coverage_llm_providers_anthropic_rs.md

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+---
+name: "Add test coverage for llm/providers/anthropic.rs (0% -> 50%+)"
+---
+
+# Story 123: Add test coverage for llm/providers/anthropic.rs
+
+Currently at 0% line coverage (204 lines). The Anthropic provider handles API communication for Claude models.
+
+## What to test
+
+- Request construction (headers, body format, model selection)
+- Response parsing (streaming chunks, tool use responses, error responses)
+- API key validation
+- Rate limit / error handling
+- Message format conversion (internal Message -> Anthropic API format)
+
+## Notes
+
+- Mock HTTP responses rather than calling the real Anthropic API
+- Use `mockito` or similar for HTTP mocking, or test the pure functions directly
+- Focus on serialization/deserialization and error paths
+
+## Acceptance Criteria
+
+- [ ] Line coverage for `llm/providers/anthropic.rs` reaches 50%+
+- [ ] Tests pass with `cargo test`
+- [ ] `cargo clippy` clean