fix(sdk): close conformance gaps in the agent, directory and service

[zhoodar]

Summary

• Correct search request serialization, individual GET defaults, compatible-minor handling, language reporting, media negotiation, and conditional responses.
• Isolate authenticated cache entries and anonymous supporting-document requests; enforce streamed response limits and close responses on failure or cancellation.
• Preserve descriptive Directory metadata without using it as catalog routing authority.
• Resolve Attribute Schema references against final retrieval URLs, ignore reference-looking literal data, and contain unusable schemas as Offering issues.
• Apply narrow capability normalization, quarantine conflicting definitions before checking merged limits, and configure resource-class cache lifetimes independently.
• Preserve search freshness policies through continuation requests.
• Enforce Service request and response limits, omit embedded versions without mutating Catalog models, and validate StaticCatalog parent relationships.
• Add public-API regressions, expand shared conformance mappings, and exercise search against the Node.js marketplace example.

Verification

• make verify on Python 3.11 and 3.14.
• make conformance; reports retain explicit skips for unmapped vectors.
• make interoperability against the Node.js static-catalog and marketplace examples, including search and continuation.

Specifications, shared conformance vectors, automatic retries, and iterator APIs are unchanged. Unmapped harness vectors remain explicitly skipped, not counted as passing.