fix(java): preserve externalizable containers in compatible mode

[mandrean]

Why?

Fory can lose entries for compatible-mode map and collection classes that implement Externalizable if they are handled as ordinary containers instead of using their externalization logic.

This fixes the data loss reported in #3621.

What does this PR do?

• Adds a regression test for an Externalizable AbstractMap stored through a Map<String, String> field.
• Routes Externalizable map and collection implementations through the JDK-compatible serializer path.
• Delegates those containers to ExternalizableSerializer so writeExternal / readExternal preserve their state.

Related issues

• Fixes COMPATIBLE mode silently drops contents of unregistered Map/Collection implementations #3621

AI Contribution Checklist

• Substantial AI assistance was used in this PR: yes
• If yes, I included a completed AI Contribution Checklist in this PR description and the required AI Usage Disclosure.
• If yes, my PR description includes the required ai_review summary and screenshot evidence of the final clean AI review results from both fresh reviewers on the current PR diff or current HEAD after the latest code changes.

AI Usage Disclosure

• substantial_ai_assistance: yes
• scope: issue investigation, regression test drafting, code change drafting, validation command selection, PR description drafting
• affected_files_or_subsystems: Java serializer resolution, compatible collection/map serializers, Java map serializer tests
• ai_review: line-by-line self-review completed locally; required two-fresh-reviewer loop is pending, so this PR remains draft until final clean review evidence can be attached
• ai_review_artifacts: pending; screenshot evidence or persisted links for both fresh final clean AI reviews will be added before marking ready for maintainer review
• human_verification:
  • Reproduced the regression by applying the new test on apache/main, where it fails with Maps do not have the same size:0 != 1.
  • Verified the fixed branch with mvn -pl fory-core -Dtest=org.apache.fory.serializer.collection.MapSerializersTest#testExternalizableMapCompatibleMode test.
  • Ran Java/style/install checks: mvn -T10 -B --no-transfer-progress spotless:check checkstyle:check, python ./ci/run_ci.py java --version 11, python ./ci/run_ci.py java --version 17, python ./ci/run_ci.py java --version 21, python ./ci/run_ci.py java --version 25, python ./ci/run_ci.py java --install-fory, and mvn -T10 --no-transfer-progress clean install -DskipTests -Dmaven.javadoc.skip=true -Dmaven.source.skip=true.
• performance_verification: N/A; correctness fix for Externalizable container serializer selection in compatible mode, no benchmark run
• provenance_license_confirmation: Apache-2.0-compatible provenance confirmed; no incompatible third-party code introduced

Does this PR introduce any user-facing change?

• Does this PR introduce any public API change?
• Does this PR introduce any binary protocol compatibility change?

Benchmark

Not run. This is a correctness fix for existing compatible-mode Externalizable map/collection handling and does not intentionally change public APIs or binary protocol format.

[chaokunyang]

LGTM