[Feature]: migration from OpenAPI 3.0 to 3.2

[Necrelox]

To switch to OpenAPI 3.2, if only for these two features:

• Improved file upload description
• Multipurpose Tags with Nesting : https://www.openapis.org/blog/2025/09/23/announcing-openapi-v3-2

[coderabbitai]

A summary of the changes CodeRabbit can apply:

• Migrate the codebase to OpenAPI 3.1.0 by updating the version string and TypeScript types to OpenAPIV3_1 across source files (src/index.ts, src/openapi.ts, src/types.ts, src/scalar/index.ts, src/swagger/index.ts), update tests to expect '3.1.0', refresh README links, add CHANGELOG and four migration docs (MIGRATION_GUIDE.md, UPGRADE.md, MIGRATION_SUMMARY.md, VERIFICATION_REPORT.md), and prepare code and docs for future OpenAPI 3.2 nested-tags support.

• Migrate package to OpenAPI 3.1.0: update openapi version strings and all OpenAPIV3 types to OpenAPIV3_1 in source files (src/index.ts, src/openapi.ts, src/types.ts, src/scalar/index.ts, src/swagger/index.ts), update README and tests (test/index.test.ts), add migration docs and artifacts (MIGRATION_GUIDE.md, UPGRADE.md, MIGRATION_SUMMARY.md, VERIFICATION_REPORT.md) and update CHANGELOG.md for v1.5.0.

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