.Net: Making Gemini MaxTokens Optional when not provided (#12539)

### Motivation and Context

- Fixes #12334

---------

Co-authored-by: Mark Wallace <[email protected]>

Author: rogerbarreto

dotnet/src/Connectors/Connectors.Google.UnitTests/GeminiPromptExecutionSettingsTests.cs

@@ -29,7 +29,7 @@ public void ItCreatesGeminiExecutionSettingsWithCorrectDefaults()
         Assert.Null(executionSettings.AudioTimestamp);
         Assert.Null(executionSettings.ResponseMimeType);
         Assert.Null(executionSettings.ResponseSchema);
-        Assert.Equal(GeminiPromptExecutionSettings.DefaultTextMaxTokens, executionSettings.MaxTokens);
+        Assert.Null(executionSettings.MaxTokens);
     }
 
     [Fact]

dotnet/src/Connectors/Connectors.Google/GeminiPromptExecutionSettings.cs

@@ -33,11 +33,6 @@ public sealed class GeminiPromptExecutionSettings : PromptExecutionSettings
     private GeminiToolCallBehavior? _toolCallBehavior;
     private GeminiThinkingConfig? _thinkingConfig;
 
-    /// <summary>
-    /// Default max tokens for a text generation.
-    /// </summary>
-    public static int DefaultTextMaxTokens { get; } = 256;
-
     /// <summary>
     /// Temperature controls the randomness of the completion.
     /// The higher the temperature, the more random the completion.
@@ -358,7 +353,7 @@ public static GeminiPromptExecutionSettings FromExecutionSettings(PromptExecutio
         switch (executionSettings)
         {
             case null:
-                return new GeminiPromptExecutionSettings() { MaxTokens = DefaultTextMaxTokens };
+                return new GeminiPromptExecutionSettings();
             case GeminiPromptExecutionSettings settings:
                 return settings;
         }