feat: add DEBUG-level logs for SPI extension loading in ExtensionLoader

[uuuyuqi]

What is the purpose of the change?

Add DEBUG-level observability to ExtensionLoader for diagnosing SPI extension loading issues without external tools.

Motivation: In environments with many internal SPI extensions across different versions, along with custom class-isolation frameworks, dependency conflicts or misconfiguration can cause extensions to be silently skipped or loaded with unexpected wrapper chains. Currently the only way to observe this is via Arthas or remote debugging, which has high operational cost. With these DEBUG logs, users can simply enable DEBUG level for org.apache.dubbo.common.extension.ExtensionLoader to get full visibility into the SPI loading process.

Changes:

• Log when extension class scanning starts and finishes (with timing, loaded names, adaptive class, wrappers)
• Log each SPI class loaded (adaptive/wrapper/extension), including source resource URL
• Log when an extension instance is created, including applied wrapper classes
• Log when an extension class is skipped due to missing @Activate(onClass=...) dependencies, with the specific missing class names
• Refactor loadClassIfActive to findMissingOnClass to return missing class names for diagnostic output

All logs are guarded by isDebugEnabled(), no performance impact in production.

Fixes #16230

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 94.44444% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 60.78%. Comparing base (d95086c) to head (8fc12fd).

Files with missing lines	Patch %	Lines
...apache/dubbo/common/extension/ExtensionLoader.java	94.44%	0 Missing and 3 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##                3.3   #16231      +/-   ##
============================================
- Coverage     60.79%   60.78%   -0.01%     
- Complexity    11751    11769      +18     
============================================
  Files          1953     1953              
  Lines         89119    89164      +45     
  Branches      13444    13454      +10     
============================================
+ Hits          54179    54199      +20     
- Misses        29369    29389      +20     
- Partials       5571     5576       +5     

Flag	Coverage Δ
integration-tests-java21	32.13% <94.44%> (-0.04%)	⬇️
integration-tests-java8	32.29% <94.44%> (+0.09%)	⬆️
samples-tests-java21	32.19% <94.44%> (+0.06%)	⬆️
samples-tests-java8	29.89% <94.44%> (+0.03%)	⬆️
unit-tests-java11	59.02% <85.18%> (-0.01%)	⬇️
unit-tests-java17	58.53% <85.18%> (-0.02%)	⬇️
unit-tests-java21	58.54% <85.18%> (+0.02%)	⬆️
unit-tests-java25	58.48% <85.18%> (+<0.01%)	⬆️
unit-tests-java8	59.02% <85.18%> (-0.04%)	⬇️

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