Why

MCP tool calls already include session_id in x-codex-turn-metadata, but descendant threads intentionally share that value with the root thread. Consumers that need to correlate work at the concrete thread level also need the current thread_id.

What changed

• add thread_id to x-codex-turn-metadata while preserving session_id as the shared session identity
• thread the two identities separately through normal turns and spawned review threads
• add regression coverage for resumed sessions, reserved metadata fields, and deferred MCP tool calls

Verification

• added focused coverage in core/src/session/tests.rs, core/src/turn_metadata_tests.rs, and core/tests/suite/search_tool.rs