.Net: Add early return if Freeze method is called on a frozen object (#…

…5057)

### Motivation and Context

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In PromptExecutionSettings is a bug which creates new ReadOnlyCollection
even object is already freezed.

### Description

<!-- Describe your changes, the overall approach, the underlying design.
These notes will help understanding how your code works. Thanks! -->

The `Freeze` method in both `PromptExecutionSettings` and
`OpenAIPromptExecutionSettings` classes now include an early return
condition. This checks if the instance is already frozen, and if so, the
method execution is immediately stopped. This improvement prevents
unnecessary computations in these classes' `Freeze` methods.

### Contribution Checklist

<!-- Before submitting this PR, please make sure: -->

- [x] The code builds clean without any errors or warnings
- [x] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [x] All unit tests pass, and I have added new tests where possible
- [x] I didn't break anyone 😄

Co-authored-by: Dmytro Struk <13853051+dmytrostruk@users.noreply.github.com>

dotnet/src/Connectors/Connectors.OpenAI/OpenAIPromptExecutionSettings.cs

@@ -186,6 +186,7 @@ public string ChatSystemPrompt
             {
                 value = DefaultChatSystemPrompt;
             }
+
             this._chatSystemPrompt = value;
         }
     }
@@ -263,12 +264,18 @@ public string ChatSystemPrompt
     /// <inheritdoc/>
     public override void Freeze()
     {
+        if (this.IsFrozen)
+        {
+            return;
+        }
+
         base.Freeze();
 
         if (this._stopSequences is not null)
         {
             this._stopSequences = new ReadOnlyCollection<string>(this._stopSequences);
         }
+
         if (this._tokenSelectionBiases is not null)
         {
             this._tokenSelectionBiases = new ReadOnlyDictionary<int, int>(this._tokenSelectionBiases);

dotnet/src/SemanticKernel.Abstractions/AI/PromptExecutionSettings.cs

@@ -74,6 +74,11 @@ public bool IsFrozen
     /// </summary>
     public virtual void Freeze()
     {
+        if (this.IsFrozen)
+        {
+            return;
+        }
+
         this._isFrozen = true;
 
         if (this._extensionData is not null)