test: add unit tests for mergeAppend, mergeSlice, and isPresent in confmap/internal

[manoj-1407]

Description

confmap/internal/merge.go contains non-trivial logic for merging configuration maps (mergeAppend, mergeSlice, isPresent) but had no test coverage.

Changes

Added confmap/internal/merge_test.go with table-driven tests covering:

• TestMergeAppend_NonOverlappingKeys — verifies both slices are merged under glob-matched keys
• TestMergeAppend_DeduplicatesSliceEntries — verifies no duplication when src entry already exists in dest
• TestMergeAppend_EmptyDest — verifies src populates an empty dest
• TestMergeAppend_EmptySrc — verifies dest is unchanged when src is empty
• TestMergeAppend_NonGlobKeyNotMergedAsSlice — verifies non-glob keys follow standard maps.Merge behavior
• TestMergeAppend_PipelineReceiversMerged — verifies pipeline receiver lists merge correctly via glob pattern
• TestMergeSlice_AppendsUniqueElements — dest elements first, unique src elements appended
• TestMergeSlice_EmptySrc / TestMergeSlice_EmptyDest / TestMergeSlice_AllDuplicates — edge cases
• TestIsPresent — table-driven across present, not present, empty slice, map value cases

Testing

cd confmap && go test ./internal/... 
ok  go.opentelemetry.io/collector/confmap/internal  0.114s

[linux-foundation-easycla]

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• ✅ login: manoj-1407 / name: manoj-1407 (f259a04)