Add default value to WP_REST_Request generic template

[Nikschavan]

Summary

• Adds a default value (array<string, mixed>) to the WP_REST_Request generic template

Problem

At PHPStan level 9, any method with a WP_REST_Request parameter that doesn't specify the generic type triggers:

Method MyController::handle_request() has parameter $request with 
generic class WP_REST_Request but does not specify its types: T

Solution

Change the template from:

@phpstan-template T of array

to:

@phpstan-template T of array = array<string, mixed>

This is backward-compatible — explicit type parameters still override the default.

Fixes #408

[szepeviktor]

Thank you.