webui: MCP Diagnostics improvements (#21803)

* Add MCP Connection diagnostics and CORS hint to web-ui

* tidy up test

* webui: Refactor and improve MCP diagnostic logging

---------

Co-authored-by: evalstate <1936278+evalstate@users.noreply.github.com>

tools/server/webui/src/lib/utils/redact.ts

@@ -0,0 +1,14 @@
+/**
+ * Redacts a sensitive value, optionally showing the last N characters.
+ *
+ * @param value - The value to redact
+ * @param showLastChars - If provided, reveals the last N characters with a leading mask
+ * @returns The redacted string
+ */
+export function redactValue(value: string, showLastChars?: number): string {
+	if (showLastChars) {
+		return `....${value.slice(-showLastChars)}`;
+	}
+
+	return '[redacted]';
+}