fix(nextjs): Inconsistent transaction naming for i18n routing (#17927)

### Problem

When using Next.js 15 App Router with `next-intl` and `localePrefix:
"as-needed"`, Web Vitals and transaction names were inconsistent across
locales:

- `/foo` (default locale, no prefix) → Transaction: `/:locale` ❌
- `/ar/foo` (non-default locale, with prefix) → Transaction:
`/:locale/foo` ✅

This caused all default locale pages to collapse into a single
`/:locale` transaction, making Web Vitals data unusable for apps with
i18n routing.

After investigation it seems like the route parameterization logic
couldn't match `/foo` (1 segment) to the `/:locale/foo` pattern (expects
2 segments) because the locale prefix is omitted in default locale URLs.

### Solution

Implemented enhanced route matching with automatic i18n prefix
detection:

1. **Route Manifest Metadata** - Added `hasOptionalPrefix` flag to route
info to identify routes with common i18n parameter names (`locale`,
`lang`, `language`)
2. **Smart Fallback Matching** - When a route doesn't match directly,
the matcher now tries prepending a placeholder segment for routes
flagged with `hasOptionalPrefix`
- Example: `/foo` → tries matching as `/PLACEHOLDER/foo` → matches
`/:locale/foo` ✓
3. **Updated Specificity Scoring** - changed route specificity
calculation to prefer longer routes when dynamic segment counts are
equal
- Example: `/:locale/foo` (2 segments) now beats `/:locale` (1 segment)

### Result

**After fix:**
```
URL: /foo         → Transaction: /:locale/foo    ✅
URL: /ar/foo      → Transaction: /:locale/foo    ✅
URL: /products    → Transaction: /:locale/products ✅
URL: /ar/products → Transaction: /:locale/products ✅
```

All routes now consistently use the same parameterized transaction name
regardless of locale, making Web Vitals properly grouped and usable.

### Backwards Compatibility

- No breaking changes - only applies when direct matching would fail
- Only affects routes with first param named `locale`/`lang`/`language`
- Non-i18n apps completely unaffected
- Direct matches always take precedence over optional prefix matching

Fixes #17775

---

Maybe we should make certain aspects of this configurable, like the
`['locale', 'lang', 'language']` collection

Author: logaretm

dev-packages/e2e-tests/test-applications/nextjs-15-intl/.gitignore

@@ -0,0 +1,51 @@
+# See https://help.github.com/articles/ignoring-files/ for more about ignoring files.
+
+# dependencies
+/node_modules
+/.pnp
+.pnp.*
+.yarn/*
+!.yarn/patches
+!.yarn/plugins
+!.yarn/releases
+!.yarn/versions
+
+# testing
+/coverage
+
+# next.js
+/.next/
+/out/
+
+# production
+/build
+
+# misc
+.DS_Store
+*.pem
+
+# debug
+npm-debug.log*
+yarn-debug.log*
+yarn-error.log*
+
+# env files (can opt-in for commiting if needed)
+.env*
+
+# vercel
+.vercel
+
+# typescript
+*.tsbuildinfo
+next-env.d.ts
+
+# Sentry
+.sentryclirc
+
+pnpm-lock.yaml
+.tmp_dev_server_logs
+.tmp_build_stdout
+.tmp_build_stderr
+event-dumps
+test-results
+

dev-packages/e2e-tests/test-applications/nextjs-15-intl/.npmrc

@@ -0,0 +1,2 @@
+@sentry:registry=http://localhost:4873
+@sentry-internal:registry=http://localhost:4873

dev-packages/e2e-tests/test-applications/nextjs-15-intl/app/[locale]/i18n-test/page.tsx

@@ -0,0 +1,9 @@
+export default async function I18nTestPage({ params }: { params: Promise<{ locale: string }> }) {
+  const { locale } = await params;
+  return (
+    <div>
+      <h1>I18n Test Page</h1>
+      <p>Current locale: {locale}</p>
+    </div>
+  );
+}

dev-packages/e2e-tests/test-applications/nextjs-15-intl/app/[locale]/page.tsx

@@ -0,0 +1,9 @@
+export default async function LocaleRootPage({ params }: { params: Promise<{ locale: string }> }) {
+  const { locale } = await params;
+  return (
+    <div>
+      <h1>Locale Root</h1>
+      <p>Current locale: {locale}</p>
+    </div>
+  );
+}

dev-packages/e2e-tests/test-applications/nextjs-15-intl/app/layout.tsx

@@ -0,0 +1,11 @@
+export const metadata = {
+  title: 'Next.js 15 i18n Test',
+};
+
+export default function RootLayout({ children }: { children: React.ReactNode }) {
+  return (
+    <html lang="en">
+      <body>{children}</body>
+    </html>
+  );
+}

dev-packages/e2e-tests/test-applications/nextjs-15-intl/i18n/request.ts

@@ -0,0 +1,14 @@
+import { getRequestConfig } from 'next-intl/server';
+import { hasLocale } from 'next-intl';
+import { routing } from './routing';
+
+export default getRequestConfig(async ({ requestLocale }) => {
+  // Typically corresponds to the `[locale]` segment
+  const requested = await requestLocale;
+  const locale = hasLocale(routing.locales, requested) ? requested : routing.defaultLocale;
+
+  return {
+    locale,
+    messages: {},
+  };
+});

dev-packages/e2e-tests/test-applications/nextjs-15-intl/i18n/routing.ts

@@ -0,0 +1,10 @@
+import { defineRouting } from 'next-intl/routing';
+import { createNavigation } from 'next-intl/navigation';
+
+export const routing = defineRouting({
+  locales: ['en', 'ar', 'fr'],
+  defaultLocale: 'en',
+  localePrefix: 'as-needed',
+});
+
+export const { Link, redirect, usePathname, useRouter } = createNavigation(routing);

dev-packages/e2e-tests/test-applications/nextjs-15-intl/instrumentation-client.ts

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+import * as Sentry from '@sentry/nextjs';
+
+Sentry.init({
+  environment: 'qa',
+  dsn: process.env.NEXT_PUBLIC_E2E_TEST_DSN,
+  tunnel: `http://localhost:3031/`,
+  tracesSampleRate: 1.0,
+  sendDefaultPii: true,
+});
+
+export const onRouterTransitionStart = Sentry.captureRouterTransitionStart;

dev-packages/e2e-tests/test-applications/nextjs-15-intl/instrumentation.ts

@@ -0,0 +1,13 @@
+import * as Sentry from '@sentry/nextjs';
+
+export async function register() {
+  if (process.env.NEXT_RUNTIME === 'nodejs') {
+    await import('./sentry.server.config');
+  }
+
+  if (process.env.NEXT_RUNTIME === 'edge') {
+    await import('./sentry.edge.config');
+  }
+}
+
+export const onRequestError = Sentry.captureRequestError;

dev-packages/e2e-tests/test-applications/nextjs-15-intl/middleware.ts

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+import createMiddleware from 'next-intl/middleware';
+import { routing } from './i18n/routing';
+
+export default createMiddleware(routing);
+
+export const config = {
+  matcher: ['/((?!api|_next|.*\\..*).*)'],
+};