Improve API documentation quality

[weilun88313]

Summary

• enrich all 28 public API operations with descriptions and schema-valid request/response examples
• keep /external/integrations/* out of the public contract and record it internally as an Agent-only exclusion
• remove unused OpenAPI response components and make Redocly warnings fail CI
• add regression tests for the 28-operation boundary, descriptions, examples, license metadata, and the integrations exclusion

Validation

• python3 -m unittest scripts/test_sync_public_assets.py
• python3 scripts/sync-public-assets.py --check
• JSON validation and byte-for-byte comparison of all three OpenAPI copies
• legacy API-key guidance scan
• Redocly 2.39.0: 0 errors, 0 warnings
• spec and standards reviews: no findings

Mintlify CLI evaluation

The pinned Mintlify CLI was evaluated but intentionally not added as a blocking check. mint validate did not complete reliably, while mint broken-links reported an OpenAPI parsing error for docs.json and still exited successfully, creating a false-green risk. The deterministic repository checks and zero-warning Redocly gate remain blocking.