fix(common): close input stream in ConfigUtils#loadProperties fallback

[LouisLau-art]

What

Fixes an input stream resource leak in ConfigUtils.loadProperties(...) when allowMultiFile == false and fallback loading path is used.

Previously the code called:

• properties.load(ClassUtils.getClassLoader().getResourceAsStream(fileName));

This could leave the stream unclosed. The fallback now uses try-with-resources and null-checks the stream before loading.

Why

This addresses issue #16098 and keeps resource handling consistent with other load paths in ConfigUtils.

Verification

• ./mvnw -pl dubbo-common -Dtest=ConfigUtilsTest test -DskipITs
• Result: BUILD SUCCESS (Tests run: 25, Failures: 0, Errors: 0, Skipped: 1)

Fixes #16098

[codecov-commenter]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 60.79%. Comparing base (2c49565) to head (7a1e4d2).
⚠️ Report is 6 commits behind head on 3.3.

Files with missing lines	Patch %	Lines
...ava/org/apache/dubbo/common/utils/ConfigUtils.java	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##                3.3   #16116      +/-   ##
============================================
- Coverage     60.80%   60.79%   -0.01%     
+ Complexity    11753    11751       -2     
============================================
  Files          1953     1953              
  Lines         89118    89120       +2     
  Branches      13444    13445       +1     
============================================
- Hits          54187    54184       -3     
+ Misses        29367    29366       -1     
- Partials       5564     5570       +6     

Flag	Coverage Δ
integration-tests-java21	32.25% <66.66%> (+0.08%)	⬆️
integration-tests-java8	32.23% <66.66%> (+<0.01%)	⬆️
samples-tests-java21	32.12% <66.66%> (-0.03%)	⬇️
samples-tests-java8	29.70% <66.66%> (-0.01%)	⬇️
unit-tests-java11	59.03% <66.66%> (-0.01%)	⬇️
unit-tests-java17	?
unit-tests-java21	58.57% <66.66%> (+0.03%)	⬆️
unit-tests-java25	58.48% <66.66%> (-0.01%)	⬇️
unit-tests-java8	59.05% <66.66%> (+<0.01%)	⬆️

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