#292 Fix OpenAPI review issues

Summary

Fixes all valid findings from inline code review:

• Relative server URL — Replace https://example.com/api/v1 and http://localhost/api/v1 with /api/v1 so generated clients point at the hosting app
• OAS 3.1 nullable — nullable: true is removed in OpenAPI 3.1; fix OptionField.default → type: ['string','null'] and Conversion.result_file → anyOf: [$ref File, type null]
• Dynamic curl URLs — Replace hardcoded https://example.com/api/v1/... with {{ url('/api/v1/...') }} across all 6 curl examples
• Pinned Redoc CDN — Change /latest/ to /2.x/ to avoid silent breakage on major version releases

Skipped nitpick: @throws docblock on openApiSpec() — test helpers intentionally let exceptions bubble; no documentation value.

Test plan

• composer test passes (620 tests)
• composer lint passes
• npm run build passes
• No nullable keyword remains in openapi.yaml

🤖 Generated with Claude Code

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Summary by cubic

Aligns the OpenAPI spec and docs with OAS 3.1 and the current host. Fixes client generation issues and prevents breakage in example commands and docs rendering.

• Bug Fixes
  • Use relative server URL /api/v1 so generated clients target the hosting app.
  • Replace OAS 3.0 nullable with OAS 3.1 unions:
    • OptionField.default → type: ['string','null']
    • Conversion.result_file → anyOf: [File, null]
  • Make curl examples dynamic via {{ url('/api/v1/...') }} instead of hardcoded https://example.com.
  • Pin the ReDoc CDN from /latest/ to /2.x/ to avoid breaking changes on major releases.