Add integration tests for cached provider results

[jdfalk]

Implement integration tests validating manual search result caching behavior.

Scope:

• Verify cache hit/miss behavior for identical queries
• Ensure cache invalidation on provider changes and relevant settings updates
• Validate TTL behavior (if applicable) and cold-start fallback
• Exercise both API endpoints (/api/search, preview) and CLI paths

Acceptance Criteria:

• Tests run in CI and pass reliably
• Coverage includes at least two providers with different result shapes
• Documented in README or testing guide

Refs: internal TODO.md item (Search: Add integration tests for cached provider results)