P0 fixes:
- ModelError.Is(): use exact matching instead of substring contains()
- shouldClearStickySession: add context param for cancellation/tracing
P1 fixes:
- TODO stubs: return 501 Not Implemented errors
- validateInstanceSignature: deduplicate to shared validateCodeSignature()
- Error messages: standardize to English only
- http.go: remove pseudo if-else with duplicate branches
da36506b89