Run model tests against both JSON mappers instead of picking one at r…

[loggk]

Description

ModelTestCase chooses between JsonbJsonpMapper and JacksonJsonpMapper with a single Random coin flip per JVM, so each test run exercises one serialization backend and never the other, chosen at random. A bug affecting only one mapper passes CI about half the time.

Changes

• Replace the random mapper selection in ModelTestCase with JUnit 4 Parameterized, running every test against both mappers. @RunWith is inherited and the mapper is injected by field, so none of the 104 subclasses need changes.

Verified with ./gradlew :java-client:unitTest: 426 tests become 604, with exactly 178 per mapper and 0 failures. Runs are now deterministic, and failures report which mapper broke (testName[json-b] / testName[jackson]). spotlessJavaCheck passes.

Issues Resolved

Resolves #770

Check List

• All tests pass
• Commits are signed per the DCO using --signoff
• Changelog updated

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