fix: strengthen protocol parsing and test coverage

[nkavian]

Expand tests for JSON round trips, additive members, response normalization, pagination, resource identities, URL boundaries, and validation errors. Exercise OpenAPI path extensions and malformed transport addresses through the existing implementations.

The tests exposed three parsing defects, corrected here:

• Omit the whole branding object when either image has an unknown type, as required by the draft, so an otherwise usable Service Document remains valid.
• Handle IPv6 addresses without passing their colons through domain-name conversion.
• Reject underscore locale strings where ODP requires RFC 5646 language tags.

Validation: make verify (including race detection), make consumer-smoke, make conformance, and git diff --check passed. Coverage exclusions and thresholds are unchanged.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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