ICU-22908 DRAFT!!! MF2 ICU4J: Update spec tests and update implementation for recent spec changes

[mihnita]

Checklist

• Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-22908
• Required: The PR title must be prefixed with a JIRA Issue number.
• Required: The PR description must include the link to the Jira Issue, for example by completing the URL in the first checklist item
• Required: Each commit message must be prefixed with a JIRA Issue number.
• Issue accepted (done by Technical Committee after discussion)
• Tests included, if applicable
• API docs and/or User Guide docs changed or added, if applicable

[mihnita]

Updated the Java code to pass the tests equivalent to #3198

The .json files are not identical, the ones here "bring back" the javaIgnore.
Many of them (close to 60) are about issue 782, which is about error handling (fallback vs strict).

That is implemented, but it is an either / or, with a flag.
And I didn't update the test framework to do that.

TLDR: if we merge the 2 PRs into one, then it should contain these updated .json files, not only the Java code changes.

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I still think it is a bad idea to try and push for this soo late in the game.

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Notice: the branch changed across the force-push!

• icu4j/main/a is now changed in the branch
• icu4j/main/core/src/main/java/com/ibm/icu/message2/MFSerializer.java is now changed in the branch
• icu4j/main/core/src/test/java/com/ibm/icu/dev/test/message2/CustomFormatterMessageRefTest.java is now changed in the branch
• icu4j/main/core/src/test/java/com/ibm/icu/dev/test/message2/CustomFormatterPersonTest.java is now changed in the branch
• icu4j/main/core/src/test/java/com/ibm/icu/dev/test/message2/MessageFormat2Test.java is now changed in the branch
• icu4j/main/core/src/test/java/com/ibm/icu/dev/test/message2/Mf2IcuTest.java is now changed in the branch
• icu4j/main/core/src/test/java/com/ibm/icu/dev/test/message2/SerializationTest.java is now changed in the branch

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[catamorphism]

Is it possible to change the data model so that selectors are constrained to be Variables rather than Expressions? (If you're trying to avoid a public API change, I understand... although that means it's possible to construct a data model that doesn't correspond to any syntactically correct message.)

[catamorphism]

See https://github.com/unicode-org/message-format-wg/blob/main/spec/message.abnf#L38