Use url type for endpoint parameter in Azure Core Data Plane Service playground sample (#3839)

The playground sample was using `endpoint: string` instead of `endpoint:
url` in the `@server` decorator, inconsistent with best practices and
other samples throughout the codebase.

## Changes

- Updated
`packages/typespec-azure-playground-website/samples/azure-core.tsp` to
use `endpoint: url`

```diff
 @server(
   "{endpoint}/widget",
   "Contoso Widget APIs",
   {
-    endpoint: string,
+    endpoint: url,
   }
 )
```

This aligns with the pattern used in documentation and test files across
the repository.

<!-- START COPILOT ORIGINAL PROMPT -->



<details>

<summary>Original prompt</summary>

> 
> ----
> 
> *This section details on the original issue you should resolve*
> 
> <issue_title>Default data plane service in playground should use a url
endpoint</issue_title>
> <issue_description>```@server(
>   "{endpoint}/widget",
>   "Contoso Widget APIs",
>   {
>     /** 
> Supported Widget Services endpoints (protocol and hostname, for
example:
> https://westus.api.widget.contoso.com).
>  */
>     endpoint: string,
>   }
> )
> ```
> Is there any reason to define this as a string? I believe the best
practice is to use `url`.
> 
> [Playground
Link](https://azure.github.io/typespec-azure/playground/?sample=Azure+Core+Data+Plane+Service&options=%7B%22linterRuleSet%22%3A%7B%22extends%22%3A%5B%22%40azure-tools%2Ftypespec-azure-rulesets%2Fdata-plane%22%5D%7D%7D&vs=%7B%7D)</issue_description>
> 
> ## Comments on the Issue (you are @copilot in this section)
> 
> <comments>
> </comments>
> 


</details>



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- Fixes #3838

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Co-authored-by: JoshLove-msft <[email protected]>

Author: Copilot

packages/typespec-azure-playground-website/samples/azure-core.tsp

@@ -27,7 +27,7 @@ using Azure.Core.Traits;
 Supported Widget Services endpoints (protocol and hostname, for example:
 https://westus.api.widget.contoso.com).
  */
-    endpoint: string,
+    endpoint: url,
   }
 )
 @versioned(Contoso.WidgetManager.Versions)