Fix #16295: JSON SPI loading without optional dependencies

[larrychen1024]

What is the purpose of the change?

Fixes #16295.

On JDK 25, ServiceLoader.Iterator#hasNext() may resolve a JSON provider's constructor and its referenced types. If an optional JSON implementation such as Gson is not present, this can throw NoClassDefFoundError before Dubbo can continue to the default fastjson2 implementation.

Brief changelog

• Keep ServiceLoader#hasNext(), provider construction, and support checks inside the guarded iteration.
• Skip provider loading failures caused by ServiceConfigurationError, LinkageError, or provider runtime exceptions.
• Avoid catching fatal JVM errors through a blanket Throwable catch.
• Add a regression test that hides all Gson classes while retaining the Gson service provider entry, and verifies that fastjson2 is still discovered.

Verifying this change

• Reproduced the original NoClassDefFoundError: com/google/gson/JsonSyntaxException locally on Temurin JDK 25.0.3 with the Dubbo 3.3.6 loading logic and Gson removed from the runtime classpath.
• Verified the fixed runtime selects FastJson2Impl without Gson on JDK 25.0.3 and JDK 26.0.1.
• JsonUtilsTest: 6 tests passed on JDK 25.0.3.
• The new missing-Gson regression test passed on JDK 25.0.3 and JDK 26.0.1.
• Spotless formatting check passed.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 60.87%. Comparing base (d0bf5c3) to head (316d0f9).

Additional details and impacted files

@@            Coverage Diff            @@
##                3.3   #16385   +/-   ##
=========================================
  Coverage     60.86%   60.87%           
- Complexity    11763    11773   +10     
=========================================
  Files          1953     1953           
  Lines         89262    89262           
  Branches      13471    13471           
=========================================
+ Hits          54331    54338    +7     
- Misses        29329    29336    +7     
+ Partials       5602     5588   -14     

Flag	Coverage Δ
integration-tests-java21	32.16% <100.00%> (+0.05%)	⬆️
integration-tests-java8	32.30% <100.00%> (+0.14%)	⬆️
samples-tests-java21	32.19% <100.00%> (+0.04%)	⬆️
samples-tests-java8	29.85% <100.00%> (+0.06%)	⬆️
unit-tests-java11	59.07% <100.00%> (-0.05%)	⬇️
unit-tests-java17	58.58% <100.00%> (-0.02%)	⬇️
unit-tests-java21	58.55% <100.00%> (-0.06%)	⬇️
unit-tests-java25	58.53% <100.00%> (-0.01%)	⬇️
unit-tests-java8	59.09% <100.00%> (+<0.01%)	⬆️

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