Follow-up from PR #1696 review.

Two low-severity test gaps identified by reviewer:

• GAP-1: parse_tool_response() positive path with AnthropicContentBlock::Compaction has no unit test
• GAP-2: split_messages_structured() MessagePart::Compaction round-trip has no unit test

These are non-blocking but should be covered for full regression safety.

Files: crates/zeph-llm/src/claude.rs, crates/zeph-llm/src/provider.rs