Summary

• Skip Honcho cache-busting config reads on gateway messages unless memory.provider is honcho.
• Memoize Honcho cache-busting fields by honcho.json mtime for active Honcho gateways.

Refs #33382

Why

• _extract_cache_busting_config() runs on the gateway message hot path before agent dispatch.
• The Honcho identity fields are only relevant when Honcho is the active memory provider, but the existing path parsed honcho.json for every provider.

Changes

• gateway/run.py: adds memory.provider to cache-busting keys, gates Honcho config parsing to the active Honcho provider, and caches extracted Honcho signature fields by (path, st_mtime_ns).
• tests/gateway/test_agent_cache.py: covers non-Honcho skip behavior and mtime-based memoization while keeping existing agent config signature tests intact.

Validation

• python -m pytest tests/gateway/test_agent_cache.py::TestExtractCacheBustingConfig tests/gateway/test_agent_cache.py::TestAgentConfigSignatureUserId -o 'addopts=' -q
• python -m pytest tests/gateway/test_agent_cache.py -o 'addopts=' -q
• python -m ruff check gateway/run.py tests/gateway/test_agent_cache.py

Scope

• In scope: avoid unnecessary Honcho disk/config parsing in the gateway cache signature path and keep active Honcho config edits detectable.
• Out of scope: moving cache-busting responsibilities into memory-provider hooks (memory: move provider-specific cache-busting keys out of gateway/run.py into a MemoryProvider hook #33381) or changing the Honcho field set beyond the existing signature keys.

Note: #33440 touches the same Honcho cache-signature block but addresses manager-scoped field coverage rather than the inactive-provider skip/memoization path here.