[Foundation] Fix nullability in NSScriptCommandDescription.

[rolfbjarne]

This is file 22 of 47 files with nullability disabled in Foundation.

Changes:

• Enabled nullable reference types
• Made description field nullable (NSScriptCommandDescriptionDictionary?)
• Made GetTypeForArgument return type nullable (string?) as it can return null
• Made Dictionary property return type nullable (NSDictionary?) with null-conditional access
• Changed private static fields to static readonly for better practices
• Replaced manual null checks with ArgumentNullException.ThrowIfNull
• Fixed typo in comment: "kyes" -> "keys" and "apple" -> "Apple"
• Replaced "To be added." XML documentation with proper documentation for all members
• Added comprehensive documentation for Create method with exception documentation
• Added see cref attributes throughout for better cross-referencing
• Removed unnecessary whitespace in XML comments while preserving correct indentation
• Added exception documentation where appropriate

Contributes towards #17285.

[Copilot]

Pull request overview

This PR enables nullable reference types for NSScriptCommandDescription in the Foundation namespace, part of a larger effort to add nullability support across 47 Foundation files. The changes properly annotate nullable types, improve validation using ArgumentNullException.ThrowIfNull, fix typos, and replace placeholder XML documentation with comprehensive descriptions.

Key changes:

• Enabled #nullable enable and added proper nullability annotations to fields, properties, and return types
• Replaced manual null checks with ArgumentNullException.ThrowIfNull following .NET 9+ conventions
• Enhanced XML documentation from placeholder "To be added." text to detailed descriptions with proper cross-references

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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 )

ℹ️ Generator diff

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

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💻 [CI Build #2db82b3] Tests on macOS X64 - Mac Sonoma (14) passed 💻

✅ All tests on macOS X64 - Mac Sonoma (14) passed.

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💻 [CI Build #2db82b3] Tests on macOS M1 - Mac Ventura (13) passed 💻

✅ All tests on macOS M1 - Mac Ventura (13) passed.

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💻 [CI Build #2db82b3] Tests on macOS arm64 - Mac Sequoia (15) passed 💻

✅ All tests on macOS arm64 - Mac Sequoia (15) passed.

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💻 [CI Build #2db82b3] Tests on macOS M1 - Mac Monterey (12) passed 💻

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💻 [CI Build #2db82b3] Tests on macOS arm64 - Mac Tahoe (26) passed 💻

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🚀 [CI Build #2db82b3] Test results 🚀

Test results

✅ All tests passed on VSTS: test results.

🎉 All 130 tests passed 🎉

Tests counts

✅ 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
✅ dotnettests (tvOS): 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 6 tests passed. Html Report (VSDrops) Download
✅ linker: All 44 tests passed. Html Report (VSDrops) Download
✅ monotouch (iOS): All 11 tests passed. Html Report (VSDrops) Download
✅ monotouch (MacCatalyst): All 15 tests passed. Html Report (VSDrops) Download
✅ monotouch (macOS): All 12 tests passed. Html Report (VSDrops) Download
✅ monotouch (tvOS): All 11 tests passed. Html Report (VSDrops) Download
✅ msbuild: All 2 tests passed. Html Report (VSDrops) Download
✅ windows: All 3 tests passed. [attempt 2] 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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