Fix fastjson2 concurrent ref deserialization

[goutamadwant]

What is the purpose of the change?

Fixes #16368.

Fastjson2 JSONB deserialization can lose nested fields when multiple threads deserialize the same $ref-heavy payload for the same target class at the same time. In the failing case, shared lists referenced by multiple child objects can become null after concurrent deserialization.

This change routes both FastJson2ObjectInput#readObject overloads through a shared helper and synchronizes the fastjson2 parse per requested target class. The lock is backed by ClassValue, so it does not retain application classes or classloaders, and unrelated DTO classes can still deserialize concurrently.

The regression test serializes an object graph with shared nested lists, verifies a single read succeeds, then repeatedly deserializes the same payload from 200 threads while clearing fastjson2 reader caches between rounds. Without the fix, the test reports a non-zero count of null nested fields.

Tests:

• Focused repro: FastJson2SerializationTest#testConcurrentReadObjectWithReferences
• Targeted slice: FastJson2SerializationTest,TypeMatchTest
• Surefire result: 807 tests, 0 failures, 0 errors, 0 skipped

Checklist

• Make sure there is a GitHub_issue field for the change.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• Write necessary unit-test to verify your logic correction.
• Make sure GitHub actions can pass.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 70.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 60.47%. Comparing base (ed0e11e) to head (f20ccc7).

Files with missing lines	Patch %	Lines
...mmon/serialize/fastjson2/FastJson2ObjectInput.java	70.00%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##                3.3   #16369      +/-   ##
============================================
- Coverage     60.86%   60.47%   -0.39%     
+ Complexity    11768    10245    -1523     
============================================
  Files          1953     1953              
  Lines         89260    89264       +4     
  Branches      13471    13470       -1     
============================================
- Hits          54329    53986     -343     
- Misses        29350    29646     +296     
- Partials       5581     5632      +51     

Flag	Coverage Δ
integration-tests-java21	?
integration-tests-java8	?
samples-tests-java21	32.15% <70.00%> (-0.01%)	⬇️
samples-tests-java8	29.72% <0.00%> (-0.11%)	⬇️
unit-tests-java11	59.10% <60.00%> (+<0.01%)	⬆️
unit-tests-java17	?
unit-tests-java21	58.56% <60.00%> (+<0.01%)	⬆️
unit-tests-java25	58.54% <60.00%> (+<0.01%)	⬆️
unit-tests-java8	59.10% <60.00%> (-0.04%)	⬇️

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