feat(content-type): add setContentTypeHeaderParser - #6927

This is a complementary or alternative escape hatch than #6926

Add a contentTypeHeaderParserFactory(defaultParser) server option for replacing Fastify’s Content-Type header parsing and canonicalization policy without replacing the addContentTypeParser body-parser registry.

const fastify = Fastify({
  contentTypeHeaderParserFactory (defaultParser) {
    return function parseContentType (headerValue) {
      if (headerValue === "application/json,application/json") {
        return defaultParser("application/json")
      }
      return defaultParser(headerValue)
    }
  }
})

The custom parser returns a structural parsed-content-type result containing isValid, isEmpty, mediaType, type, subtype, parameters, and toString(). TypeScript implementations can use the exported FastifyParsedContentType interface.

• default behavior and strict rejection remain unchanged when the option is absent
• It allows for easy extension or replacement of the default parser/validator.

#6926 disables strict validation and lets raw invalid headers reach RegExp/catch-all body parsers. This proposal keeps the validation lifecycle intact but makes the header interpretation policy replaceable. Either or both could land depending on what people think.

Checklist

• run npm run test && npm run benchmark --if-present
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct