feat(directory): support source-aware directory discovery

[nkavian]

Summary

• Support source-aware mixed Directory results and source filters for search and suggestions.
• Preserve exact discovery URLs and future source formats without treating imported Services or Collections as native ODP targets.
• Update the discovery example, public documentation, and consumer coverage.

Compatibility

Mixed results use DirectoryIndexedService with required source metadata. Native Service-only search and Agent execution retain their existing contracts. Explicit ServiceFilters struct literals need the sources field or ..Default::default().

This does not implement OpenAPI execution or publish a release.