.Net: Switch from hardcoded model id (#11234)

### Motivation and Context

Small fix from the bug bash

### Description

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- [ ] The code builds clean without any errors or warnings
- [ ] The PR follows the [SK Contribution
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- [ ] All unit tests pass, and I have added new tests where possible
- [ ] I didn't break anyone 😄

Author: markwallace-microsoft

dotnet/samples/GettingStartedWithAgents/AzureAIAgent/Step01_AzureAIAgent.cs

@@ -19,7 +19,7 @@ public async Task UseTemplateForAzureAgentAsync()
         string generateStoryYaml = EmbeddedResource.Read("GenerateStory.yaml");
         PromptTemplateConfig templateConfig = KernelFunctionYaml.ToPromptTemplateConfig(generateStoryYaml);
         // Instructions, Name and Description properties defined via the PromptTemplateConfig.
-        Azure.AI.Projects.Agent definition = await this.AgentsClient.CreateAgentAsync("gpt-4o", templateConfig.Name, templateConfig.Description, templateConfig.Template);
+        Azure.AI.Projects.Agent definition = await this.AgentsClient.CreateAgentAsync(TestConfiguration.AzureAI.ChatModelId, templateConfig.Name, templateConfig.Description, templateConfig.Template);
         AzureAIAgent agent = new(
             definition,
             this.AgentsClient,