[Server] McpResourceTemplate throws 500 error when uriTemplate variable is undefined #469

Describe the bug

When defining an McpResourceTemplate with a uriTemplate that does not define the variable used by the resource template, the application throws a 500 Internal Server Error.

The error appears to be related to Symfony's response handling:

Warning: Cannot modify header information - headers already sent by (output started at /vendor/symfony/http-foundation/Response.php:393) (500 Internal Server Error)

To Reproduce

Steps to reproduce the behavior:

1. Define an McpResourceTemplate.
2. Use a variable in the resource template that is not defined in the uriTemplate.
3. Execute/request the resource.
4. The application returns a 500 Internal Server Error.

Example

  #[McpResourceTemplate(
        uriTemplate: 'data://tags',
        name: 'all_tags',
        title: 'All Tags ',
        description: 'All Tags',
        mimeType: 'application/json'
    )]
    public function tag_all( ?int $paged = 1 ) : array|Error {

        //.....

    }

Expected behavior

The application should handle an undefined or missing uriTemplate variable gracefully, ideally by returning a clear validation or configuration error rather than triggering a 500 Internal Server Error caused by headers being sent prematurely.

Logs

Warning: Cannot modify header information - headers already sent by (output started at /vendor/symfony/http-foundation/Response.php:393) (500 Internal Server Error)

Additional context

I encountered this while working with McpResourceTemplate.

I was unable to report the issue through GitHub's usual bug-reporting flow, so I am reporting it here directly.