story-kit: queue 120_story_test_coverage_llm_chat_rs for QA

.story_kit/work/2_current/120_story_test_coverage_llm_chat_rs.md

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-name: "Add test coverage for llm/chat.rs (2.6% -> 60%+)"
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-
-# Story 120: Add test coverage for llm/chat.rs
-
-Currently at 2.6% line coverage (343 lines, 334 missed). This is the chat completion orchestration layer — the biggest uncovered module by missed line count.
-
-## What to test
-
-- Message construction and formatting
-- Token counting/estimation logic
-- Chat session management
-- Error handling paths (provider errors, timeout, malformed responses)
-- Any pure functions that don't require a live LLM connection
-
-## Notes
-
-- Mock the LLM provider trait/interface rather than making real API calls
-- Focus on the logic layer, not the provider integration
-- Target 60%+ line coverage
-
-## Acceptance Criteria
-
-- [ ] Line coverage for `llm/chat.rs` reaches 60%+
-- [ ] Tests pass with `cargo test`
-- [ ] `cargo clippy` clean