fix: allow protocol related cryptographic assets in XML

[CAOShurong]

Summary

• allow XML protocolProperties to use the preferred relatedCryptographicAssets representation already available in JSON Schema and Protobuf
• extend the full cryptography fixtures in XML, JSON, and textproto so the field stays covered across formats

Fixes #1018.

Verification

• reproduced the regression before the XSD change: the Java XML suite reported only 1.7/valid-cryptography-full-1.7.xml failing
• mvn clean test (769 tests)
• npm test on Linux with Node 24
• Buf 1.58.0 lint, remote and adjacent-version breaking checks, plus all 193 valid textproto fixtures
• PHP JSON/XML schema scripts for every supported version
• docgen/xml/gen.sh for XML documentation versions 1.0 through 1.7; the generated 1.7 reference includes the new element

AI assistance

OpenAI Codex assisted with repository research, implementation, and test execution. I reviewed the change and its generated output, reproduced the failure before the fix, ran the validations above, and take responsibility for the contribution.

[CAOShurong]

Closing this in favor of #1019, which predates this PR and implements the same schema fix with clearer cryptoRef deprecation text.

I independently tested #1019 at 63cda281ab834726ca1b80c480fa9edf9693d4f0 (769 Java tests, the full Node 24 JSON suite, Buf schema lint, and the modified 1.7 textproto fixture) and submitted an approving review there. Keeping both PRs open would only duplicate maintainer work.