feat(directory): support source-aware directory discovery

[nkavian]

Summary

• Expose exact discovery sources for mixed Service and Collection results, including OpenAPI and unknown future formats.
• Support source filters in search and suggestions while preserving native ODP Service-only behavior.
• Keep imported metadata optional and prevent imported Collections from being passed to ODP operations in the discovery example.
• Update Directory documentation, consumer examples and contract tests.

Mixed results require the deployed Directory source metadata. OpenAPI document execution, CLI changes and SDK publication are outside this PR.