fix: complete methods for classes loaded by multiple ClassLoaders

[DragonFSKY]

Summary

• collect method completion candidates from same-FQCN classes loaded by different ClassLoaders
• keep ambiguous class-pattern matches with different class names from returning mixed method candidates
• add regression coverage for method completion across multiple ClassLoaders

Root Cause

completeMethodName() treated multiple matched classes as ambiguous and returned no method candidates. When the same FQCN is loaded by multiple ClassLoaders, watch/trace method completion could therefore miss available methods.

Tests

• PASS: ./mvnw -pl core -Dtest=CompletionUtilsTest test
• PASS: git diff --check

Notes

• This only changes method completion candidate collection for duplicate same-name class matches.
• It does not change class search semantics or command execution behavior.
• Broader local core/reactor checks were blocked by local JDK/native artifact setup; CI will run the full project matrix.

Fixes #2943

[CLAassistant]

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