Updates SEP-2549 caching traceability text to match
modelcontextprotocol/modelcontextprotocol#2890:
- Scope the five *-caching-hints requirements to results with
resultType: "complete" (interim input_required results carry no
hints).
- Add two excluded client MUST NOT rows from the new Cache Key
section (cache-key mismatch, multi round-trip retry results).
Neither is wire-observable since a conformant client may simply
never cache.
No check IDs change; traceability.json will be refreshed by the
scheduled traceability workflow.
Claude-Session: https://claude.ai/code/session_01Gbqx5LodtAghT5fRYFxeja
