[Foundation] Fix nullability in NSDirectoryEnumerator.

[rolfbjarne]

This is file 4 of 47 files with nullability disabled in Foundation.

Also (by Copilot):

• Improve XML documentation and nullability in NSDirectoryEnumerator.
• Remove placeholder 'To be added.' comments and replace with proper documentation.
• Remove unnecessary whitespace after triple-slash in XML comments.
• Add proper attributes for type references.
• Add comprehensive documentation for all IEnumerator implementations.
• Document exception conditions for Reset and Current properties.

Contributes towards #17285.

[vs-mobiletools-engineering-service2]

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✅ API diff for current PR / commit

NET (empty diffs)

• iOS: vsdrops gist ( No breaking changes )
• tvOS: vsdrops gist ( No breaking changes )
• MacCatalyst: vsdrops gist ( No breaking changes )
• macOS: vsdrops gist ( No breaking changes )

✅ API diff vs stable

NET (empty diffs)

• iOS: vsdrops gist ( No breaking changes )
• tvOS: vsdrops gist ( No breaking changes )
• MacCatalyst: vsdrops gist ( No breaking changes )
• macOS: vsdrops gist ( No breaking changes )

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Generator Diff: vsdrops (html) vsdrops (raw diff) gist (raw diff) - Please review changes)

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🔥 [CI Build #fa70077] Build failed (Build macOS tests) 🔥

Build failed for the job 'Build macOS tests' (with job status 'Failed')

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🔥 [CI Build #fa70077] Test results 🔥

Test results

❌ Tests failed on VSTS: test results

0 tests crashed, 1 tests failed, 114 tests passed.

Failures

❌ dotnettests tests (tvOS)

1 tests failed, 0 tests passed.

• DotNet tests: Failed (Execution failed with exit code 1)

Html Report (VSDrops) Download

Successes

✅ cecil: All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (iOS): All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (MacCatalyst): All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (macOS): All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (Multiple platforms): All 1 tests passed. Html Report (VSDrops) Download
✅ framework: All 2 tests passed. Html Report (VSDrops) Download
✅ fsharp: All 4 tests passed. Html Report (VSDrops) Download
✅ generator: All 5 tests passed. Html Report (VSDrops) Download
✅ interdependent-binding-projects: All 4 tests passed. Html Report (VSDrops) Download
✅ introspection: All 4 tests passed. Html Report (VSDrops) Download
✅ linker: All 44 tests passed. Html Report (VSDrops) Download
✅ monotouch (iOS): All 8 tests passed. Html Report (VSDrops) Download
✅ monotouch (MacCatalyst): All 11 tests passed. Html Report (VSDrops) Download
✅ monotouch (macOS): All 9 tests passed. Html Report (VSDrops) Download
✅ monotouch (tvOS): All 8 tests passed. Html Report (VSDrops) Download
✅ msbuild: All 2 tests passed. Html Report (VSDrops) Download
✅ windows: All 3 tests passed. Html Report (VSDrops) Download
✅ xcframework: All 4 tests passed. Html Report (VSDrops) Download
✅ xtro: All 1 tests passed. Html Report (VSDrops) Download

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Hash: fa700771c7ccda69ba0ee18deabb88dd6ca527dd [PR build]