Add Phase 2 test coverage for infrastructure modules:
- state.rs: 6 tests (defaults, serde round-trips, UserState deref/clone)
- provider_client.rs: 2 tests (ProviderMessage serde)
- llm.rs: 12 tests (FollowUpMessage serde, joined_len, parse_article_html
extraction with article/main/role=main tags, fallback, exclusions,
truncation, fragment skipping)
- chat.rs: 17 tests (doc_to_chat_session, doc_to_chat_message BSON
conversion, resolve_provider_url for all providers)
Refactor: extract parse_article_html from fetch_article_text for testability
without HTTP. Refactor resolve_provider_url to accept explicit params
instead of full ServerState, avoiding need for MongoDB in tests.
Total test count: 129 (up from 92).
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
d4274387a9