[Foundation] Fix nullability in NSDictionary<TKey,TValue>. (#24457)

This is file 41 of 47 files with nullability disabled in Foundation.

* Removes SupportedOSPlatform attributes without version numbers.
* Enables nullability and adds nullability attributes where appropriate.
* Replaces 'To be added' XML comments with proper documentation.
* Adds missing XML documentation for all public members.
* Uses ArgumentNullException.ThrowIfNull consistently.
* Uses modern C# array syntax ([]) for empty arrays.
* Improves IDictionary interface implementation to properly handle missing keys.
* Adds some tests.

Contributes towards #17285.

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Co-authored-by: GitHub Copilot <[email protected]>

Author: rolfbjarne