Harden media type negotiation and preserve routed params

[itsviseph]

Summary

This PR improves a couple of request/response and routing edge cases in Typhon.

Changes

• hardens media type parsing for protobuf request/response handling
• supports protobuf content types with parameters like
• avoids nil request access when encoding standalone protobuf responses
• captures router params at dispatch time and stores them in request context
• makes resilient even if request paths are mutated later in the request lifecycle

Validation

• Ran ok github.com/monzo/typhon 0.321s
  ? github.com/monzo/typhon/examples [no test files]
  ? github.com/monzo/typhon/legacyprototest [no test files]
  ? github.com/monzo/typhon/prototest [no test files]

Notes

• I focused on targeted library behavior fixes and added tests for the new cases.