[UIKit] Enable nullability and clean up UIAccessibilityCustomAction.

[rolfbjarne]

This is file 2 of 30 files with nullability disabled in UIKit.

• Enable nullability.
• Use ArgumentNullException.ThrowIfNull() instead of manual null check.
• Add nullable annotation (object?).
• Improve XML documentation comments: remove 'To be added.' placeholders, fix typos, add missing docs, fix formatting.

Contributes towards #17285.

[Copilot]

Pull request overview

Enables nullable reference types in UIAccessibilityCustomAction and updates the helper constructor/dispatcher to use modern null-checking while cleaning up XML documentation.

Changes:

• Enabled #nullable and updated the action backing field to object?.
• Replaced a manual null-check with ArgumentNullException.ThrowIfNull.
• Improved XML documentation comments (removed placeholders, fixed formatting/typos).

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✅ [PR Build #cb80c4f] Build passed (Detect API changes) ✅

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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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Hash: cb80c4f350206973314bf17bd6719760753e24ce [PR build]

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✅ [CI Build #cb80c4f] Build passed (Build macOS tests) ✅

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Hash: cb80c4f350206973314bf17bd6719760753e24ce [PR build]

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💻 [CI Build #cb80c4f] Tests on macOS M1 - Mac Monterey (12) passed 💻

✅ All tests on macOS M1 - Mac Monterey (12) passed.

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Hash: cb80c4f350206973314bf17bd6719760753e24ce [PR build]

[vs-mobiletools-engineering-service2]

💻 [CI Build #cb80c4f] Tests on macOS arm64 - Mac Sequoia (15) passed 💻

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

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Hash: cb80c4f350206973314bf17bd6719760753e24ce [PR build]

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💻 [CI Build #cb80c4f] Tests on macOS M1 - Mac Ventura (13) passed 💻

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

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💻 [CI Build #cb80c4f] Tests on macOS X64 - Mac Sonoma (14) passed 💻

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

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💻 [CI Build #cb80c4f] Tests on macOS arm64 - Mac Tahoe (26) passed 💻

✅ All tests on macOS arm64 - Mac Tahoe (26) passed.

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🚀 [CI Build #cb80c4f] Test results 🚀

Test results

✅ All tests passed on VSTS: test results.

🎉 All 117 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 4 tests passed. Html Report (VSDrops) Download
✅ linker: All 44 tests passed. Html Report (VSDrops) Download
✅ monotouch (iOS): All 9 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 9 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: cb80c4f350206973314bf17bd6719760753e24ce [PR build]