fix(schema): add missing error codes to enums/error-code.json

[garvitkaushik-123]

Summary

• Adds five error codes to enums/error-code.json that are already referenced in documentation and server implementation but were absent from the canonical enum:
  • INVALID_PRICING_OPTION — used in report_usage.mdx, create_media_buy.mdx, and report-usage-response.json examples
  • INVALID_USAGE_DATA — used in report_usage.mdx error table and task-handlers.ts validation logic
  • DUPLICATE_REQUEST — used in report_usage.mdx error table
  • TARGETING_TOO_NARROW — used in create_media_buy.mdx error table
  • CREATIVE_ID_EXISTS — used in create_media_buy.mdx and update_media_buy.mdx error tables
• Each code includes enumDescriptions (prose with recovery classification) and enumMetadata (structured recovery + suggestion) so SDKs can consume them like any other standard code

Closes #5892

Test plan

• JSON is valid (python3 -c "import json; json.load(...)")
• All 97 enum values have matching enumDescriptions and enumMetadata entries (no orphans in either direction)
• CI schema validation passes
• Changeset included for patch release

[aao-secretariat]

Argus is not auto-reviewing this PR because it modifies protected paths that require human review (static/schemas/source/enums/error-code.json). A human reviewer should review and merge this PR; Argus will resume on subsequent PRs once these changes land on main.

[bokelley]

Thanks for chasing the drift. This cannot land as a 3.1 patch: #5892 explicitly deferred the error-vocabulary decision to 3.2, and DUPLICATE_REQUEST was rejected in that disposition. The current branch also fails the drift-disposition checks. Please rework this against the recorded 3.2 vocabulary decision, remove rejected codes, and update the canonical docs and enum together rather than treating all current prose references as normative.