Avoid double-encoding of TEI, PURL, and mediaType query parameters

## Summary

Some OpenAPI query-parameter examples for tei, purl, and artifact mediaType appear to contain values that have already been percent-encoded for URI transport.

## Current issues

- `/discovery` `tei` / `purl` examples use forms like `urn%3Atei%3A...` and `pkg%3Amaven%2F...`
- Artifact / signature `mediaType` examples use `application%2Fvnd.cyclonedx%2Bjson`
- Descriptions tell callers to “provide a URL-encoded string,” which conflates the parameter value with HTTP serialization

## Proposed direction

- Use raw TEI, PURL, and media-type values in parameter schemas/examples
- State that clients apply query-parameter serialization once, and servers interpret after one query-decoding step (preserving escapes that are part of a PURL value)
- Show percent-encoded forms only as example request lines after serialization

Does this direction look right?