Skip diagnostics for non-open md files (#152687)

Currently we only show diagnostics for opened tabs. This means we shouldn't waste time computing diagnostics for these non open files

extensions/markdown-language-features/src/test/diagnostic.test.ts

@@ -75,6 +75,7 @@ class MemoryDiagnosticConfiguration implements DiagnosticConfiguration {
 }
 
 class MemoryDiagnosticReporter extends DiagnosticReporter {
+
 	private readonly diagnostics = new ResourceMap<readonly vscode.Diagnostic[]>();
 
 	override dispose(): void {
@@ -91,6 +92,10 @@ class MemoryDiagnosticReporter extends DiagnosticReporter {
 		this.diagnostics.set(uri, diagnostics);
 	}
 
+	areDiagnosticsEnabled(_uri: vscode.Uri): boolean {
+		return true;
+	}
+
 	delete(uri: vscode.Uri): void {
 		this.diagnostics.delete(uri);
 	}