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/tests/unit/redact.test.ts

@@ -0,0 +1,20 @@
+import { describe, expect, it } from 'vitest';
+import { redactValue } from '$lib/utils/redact';
+
+describe('redactValue', () => {
+	it('returns [redacted] by default', () => {
+		expect(redactValue('secret-token')).toBe('[redacted]');
+	});
+
+	it('shows last N characters when showLastChars is provided', () => {
+		expect(redactValue('session-abc12', 5)).toBe('....abc12');
+	});
+
+	it('handles value shorter than showLastChars', () => {
+		expect(redactValue('ab', 5)).toBe('....ab');
+	});
+
+	it('returns [redacted] when showLastChars is 0', () => {
+		expect(redactValue('secret', 0)).toBe('[redacted]');
+	});
+});