Add GetValueOrNull on IDictionary<,> and IReadOnlyDictionary<,> for struct values

[Copilot]

Adds GetValueOrNull extension methods for generic dictionary interfaces constrained to where TValue : struct, returning TValue? instead of default(TValue) — the nullable counterpart to GetValueOrDefault.

• IDictionary<TKey, TValue> — available on all target frameworks
• IReadOnlyDictionary<TKey, TValue> — guarded with #if NET40_OR_GREATER || NETSTANDARD || NET (not available in net35)

IDictionary<string, int> dict = new Dictionary<string, int> { ["a"] = 1 };

int? found    = dict.GetValueOrNull("a");  // 1
int? notFound = dict.GetValueOrNull("z");  // null

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.85%. Comparing base (76a2635) to head (7ef1d83).
⚠️ Report is 1 commits behind head on prime.

Additional details and impacted files

@@            Coverage Diff             @@
##            prime      #80      +/-   ##
==========================================
+ Coverage   93.83%   93.85%   +0.02%     
==========================================
  Files         134      136       +2     
  Lines        2707     2717      +10     
  Branches      215      217       +2     
==========================================
+ Hits         2540     2550      +10     
  Misses        123      123              
  Partials       44       44              

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[Copilot]

Pull request overview

Adds nullable-returning dictionary lookup helpers for struct values, providing a clearer “missing key” signal than default(TValue) and aligning with existing “OrNull” patterns in PowerKit.

Changes:

• Add GetValueOrNull for IDictionary<TKey, TValue> where TValue : struct.
• Add GetValueOrNull for IReadOnlyDictionary<TKey, TValue> where TValue : struct, guarded for non-net35 TFMs.
• Add/extend unit tests covering found vs. missing key behavior for both interfaces.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
PowerKit/Extensions/ReadOnlyDictionaryExtensions.cs	New extension method for IReadOnlyDictionary<,> returning TValue? (TFM-guarded).
PowerKit/Extensions/DictionaryExtensions.cs	Adds GetValueOrNull for IDictionary<,> returning TValue?.
PowerKit.Tests/Extensions/ReadOnlyDictionaryExtensionsTests.cs	New tests for IReadOnlyDictionary<,>.GetValueOrNull.
PowerKit.Tests/Extensions/DictionaryExtensionsTests.cs	Adds tests for IDictionary<,>.GetValueOrNull.

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