Error CS9293 was too generic when extension parameters were referenced from static members, giving "Cannot use extension parameter in this context" without explaining the static/instance mismatch.

Changes

• New error CS9347: "Static members cannot refer to extension parameter '{0}'. Use an instance member instead."
• Modified Binder_Expressions.cs to detect static member context and emit the specific error with simplified logic using extracted boolean variables for clarity
• Error message format: The parameter name is shown without its type (e.g., 'p' instead of 'int p') for conciseness
• Preserved CS9293 for other invalid contexts (parameter defaults, attributes, nested types)
• Updated 13 existing tests to expect the new error where applicable

Example

public static class Extensions
{
    extension(object o)
    {
        static object M() => o;  // Now: CS9347 (was: CS9293)
        void M2(int x = o) { }   // Still: CS9293 (correct context)
    }
}

Before: "Cannot use extension parameter 'object o' in this context."
After: "Static members cannot refer to extension parameter 'o'. Use an instance member instead."

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.