fix(common): decode URL-encoded mw_diag cookie before parsing (#43606)

## Problem

The Phase 1 mw_diag monitoring dashboard wasn't showing any data despite
the middleware PR (#43413) being live in production.

Next.js `response.cookies.set()` URL-encodes cookie values, so
`hit=1&would_stamp=0&has_cookie=0` becomes
`hit%3D1%26would_stamp%3D0%26has_cookie%3D0` in `document.cookie`. Our
`parseMwDiagCookie` was passing that encoded string directly to
`URLSearchParams`, which splits on literal `&` and `=` — the encoded
`%26`/`%3D` delimiters were invisible to it. Result: `params.get('hit')`
always returned `null`, so every diagnostic cookie was silently dropped.

## Changes

- Add `decodeURIComponent()` before `URLSearchParams` in
`parseMwDiagCookie`
- Add 3 test cases covering URL-encoded cookie values (matching what
Next.js actually produces in production)

## Testing

- All 34 unit tests pass (31 existing + 3 new)
- Verified locally with Playwright: manually set the URL-encoded cookie
format on localhost:8082, confirmed `parseMwDiagCookie` now returns
`{hit: true, ...}` and the data flows through to `PageTelemetry`

GROWTH-625

packages/common/first-referrer-cookie.test.ts

@@ -232,6 +232,33 @@ describe('first-referrer-cookie', () => {
         has_existing_cookie: false,
       })
     })
+
+    it('parses URL-encoded value from Next.js response.cookies.set()', () => {
+      const header = `${MW_DIAG_COOKIE_NAME}=hit%3D1%26would_stamp%3D1%26has_cookie%3D0`
+      expect(parseMwDiagCookie(header)).toEqual({
+        hit: true,
+        would_stamp: true,
+        has_existing_cookie: false,
+      })
+    })
+
+    it('parses URL-encoded value with has_cookie=1', () => {
+      const header = `${MW_DIAG_COOKIE_NAME}=hit%3D1%26would_stamp%3D0%26has_cookie%3D1`
+      expect(parseMwDiagCookie(header)).toEqual({
+        hit: true,
+        would_stamp: false,
+        has_existing_cookie: true,
+      })
+    })
+
+    it('parses URL-encoded value among multiple cookies', () => {
+      const header = `session=abc; ${MW_DIAG_COOKIE_NAME}=hit%3D1%26would_stamp%3D0%26has_cookie%3D0; theme=dark`
+      expect(parseMwDiagCookie(header)).toEqual({
+        hit: true,
+        would_stamp: false,
+        has_existing_cookie: false,
+      })
+    })
   })
 
   describe('shouldRefreshCookie', () => {

packages/common/first-referrer-cookie.ts

@@ -290,8 +290,8 @@ export function parseMwDiagCookie(cookieHeader: string): MwDiagData | null {
 
     if (!match) return null
 
-    const value = match.slice(`${MW_DIAG_COOKIE_NAME}=`.length)
-    const params = new URLSearchParams(value)
+    const rawValue = match.slice(`${MW_DIAG_COOKIE_NAME}=`.length)
+    const params = new URLSearchParams(decodeURIComponent(rawValue))
 
     if (params.get('hit') !== '1') return null