Add parsers for common content types.

[samuel-williams-shopify]

Summary

• add bounded JSON, URL-encoded form, and multipart form representation parsers
• provide an explicitly required, frozen Protocol::Content::Parser.default registry
• preserve custom result and incremental enumeration interfaces for form parsers
• keep format libraries optional and load adapters only through their dedicated entry points
• bound JSON documents to 2 MiB and 32 nesting levels by default using size_limit: and depth_limit:

The JSON limits can be disabled independently with nil. Multipart uploads remain explicitly streaming and require MultipartFormParser#each or a #parse block; the default does not impose tempfile semantics.

Depends on socketry/protocol-url#4 and socketry/protocol-multipart#6 being merged and released.

Verification

Tested locally against both sibling PR branches:

• 34 tests, 51 assertions
• 100% line coverage
• RuboCop clean
• Decode: 30/30 public definitions documented
• gem package builds with only protocol-media as a runtime dependency