Inherit allow_zero_lintable_files from parent config

[shubhransh-gupta]

Summary

Resolves #6240.

When merging a parent configuration with a child configuration via parent_config, Configuration+Merging.swift was taking childConfiguration.allowZeroLintableFiles. Because allow_zero_lintable_files defaults to false when unspecified in the child config dictionary, the child config was inadvertently overriding and discarding allow_zero_lintable_files: true defined in the parent config.

Changes

• In Configuration+Merging.swift, merge allowZeroLintableFiles using childConfiguration.allowZeroLintableFiles || allowZeroLintableFiles.
• Added unit test allowZeroLintableFilesMerging in Tests/FileSystemAccessTests/ConfigurationTests.swift.
• Added CHANGELOG.md entry under Bug Fixes following repository guidelines.

[SwiftLintBot]

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[SimplyDanny]

Did you check the history of #6240? The issue thread, open PR, comments, ...? The implementation requires more than what you and others proposed and requires careful consideration.

[shubhransh-gupta]

Thanks for pointing that out, @SimplyDanny.

I reviewed the history across #6240, #5724, and the open draft in #6874. You're completely right:

1. A localized || merge is flawed because it prevents a child configuration from explicitly overriding a parent setting with allow_zero_lintable_files: false.
2. The same implicit default overwrite issue affects all scalar options (strict, lenient, indentation, baseline, write_baseline, check_for_updates), so handling only allow_zero_lintable_files creates inconsistent merge behavior.
3. PR Fix parent-child scalar configuration inheritance #6874 already provides the proper explicit-key provenance tracking to address this across all scalar options.

Closing this PR in favor of #6874.