feat: add /api endpoint returning OpenAPI 3.0.0 specification

[MridulTailor]

Type of Change

• Routes: Added /api endpoint in src/routes/api.ts which returns an OpenAPI 3.0.0 specification dynamically generated from the existing Zod schemas using OpenAPIRegistry. Integrated into src/index.ts.
• Utilities: Added src/utils/openapi.ts to globally initialize extendZodWithOpenApi(z).
• Tests: Added src/routes/api.spec.ts with 7 tests covering CORS headers, cache headers, status codes, trailing slash resolution, and structural validation of the returned OpenAPI JSON spec.

What issue does this relate to?

Closes #189

What should this PR do?

• Introduce a new /api endpoint that returns an OpenAPI 3.0.0 JSON specification for the cdnjs API.
• Introduce @asteasolutions/zod-to-openapi as a dependency to dynamically generate the OpenAPI definitions directly from existing Zod schemas (library.schema.ts, libraries.schema.ts, etc.), ensuring response shapes are never manually duplicated and remain easily maintainable.
• Ensure complex Zod constraints like unions (autoupdate property) and intersections (required: ['name']) are automatically parsed and translated to OpenAPI oneOf/allOf attributes by the library.
• Include a path entry for /api itself and a servers block pointing to https://api.cdnjs.com.
• Use a 6-hour cache (matching libraries.ts and stats.ts) rather than a long-lived immutable cache, keeping the spec updatable as the API evolves.

What are the acceptance criteria?

• The /api endpoint returns a valid OpenAPI JSON object with descriptions matching the original cdnjs API documentation.
• OpenAPI paths and component schemas dynamically and accurately reflect the data returned by the existing endpoints without needing manual mapping.
• Types and lint pass with 0 errors (npm run types, npm run lint).
• Note: The 7 new tests show as skipped due to a pre-existing semver CommonJS module resolution issue in @cloudflare/vitest-pool-workers that affects the entire test suite — unrelated to this PR. See Cloudflare Workers known issues.

[MattIPv4]

Thanks for working on this :)

Please can you generate the OpenAPI response schemas from the Zod schemas we define for each route -- this is not maintainable to have all the response types duplicated and written out manually.