Adds two boundary tests suggested by code review:
- TestBuildProviderRequest_CachingEnabled_EmptyMessages: verifies
that caching with an empty message list still emits a CacheHints
with LastCacheableMessageIndex=-1, not a spurious breakpoint.
- TestBuildProviderRequest_CachingNonNilButDisabled: verifies that
an explicitly-disabled cacheConfig (non-nil, enabled=false)
produces nil CacheHints, exercising the &&-guard branch that
the previous "disabled" test left untested.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Steve Dudenhoeffer
01b18dcf32