.Net: [HB Planner] Content type fix (#5133)

### Motivation and Context

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Resolves #5127: If the Content-Type header of a REST API JSON response
includes a charset (e.g. Content-Type: application/json; charset=utf-8),
the Handlebars prompt template handles the response as a string instead
of as deserialized JSON.

This PR changes the check from strict equality to a check that determine
if the content type simply contains `application/json`

### 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 😄

dotnet/src/Extensions/PromptTemplates.Handlebars/Helpers/KernelHelpers/KernelFunctionHelpers.cs

@@ -211,7 +211,7 @@ private static void ProcessPositionalArguments(KernelFunctionMetadata functionMe
         if (resultAsObject is RestApiOperationResponse restApiOperationResponse)
         {
             // Deserialize any JSON content or return the content as a string
-            if (string.Equals(restApiOperationResponse.ContentType, "application/json", StringComparison.OrdinalIgnoreCase))
+            if (restApiOperationResponse.ContentType?.IndexOf("application/json", StringComparison.OrdinalIgnoreCase) >= 0)
             {
                 var parsedJson = JsonValue.Parse(restApiOperationResponse.Content.ToString());
                 return KernelHelpersUtils.DeserializeJsonNode(parsedJson);