feat(directory): add mixed service and collection discovery

[nkavian]

Summary

• Add mixed Service and Collection search, continuation support, and filtered name suggestions.
• Send mixed suggestions through POST with search filters; retain the separate Service-only GET suggestions API.
• Represent Collection identity, indexing timestamps, Service attribution, and unknown future result types.
• Update public documentation, examples, and coverage while keeping Agent federation Service-only.
• Register Jackson 2 Java time support for Directory timestamps without changing the Jackson or schema-validator version families.

Compatibility

Preserves the existing Service-only methods and adds mixed search methods.

The mixed methods require the Directory server's new search and suggestions endpoints. Deploy the server before switching consumers. CLI integration and publication are separate work.