fix: match bare basePath root for catch-all middleware matchers

[Xavrir]

What

When basePath is set, middleware did not run on the bare basePath root. With

// middleware config
export const config = {
  matcher: ['/((?!api|_next/static|_next/image|favicon.ico).*)'],
}

and basePath: '/docs' in next.config, a request to /docs skipped
middleware, while /docs/ and /docs/foo ran it. Without a basePath the same
matcher runs on /. Fixes #73786.

Why

getMiddlewareMatchers in packages/next/src/build/analysis/get-page-static-info.ts
builds each matcher's source and then prepends basePath. A catch-all like
/((?!api).*) matches / because its one capture group can be empty, so the
compiled regex is ^(?:/(...))(...)?[/#?]?$ and / matches.

Once basePath is prepended the regex becomes ^/docs(?:/(...))..., which
requires a slash right after /docs. The bare /docs then fails to match even
though / matched before. The matcher is baked into the middleware manifest, so
this affects both the dev server and production (including Vercel), not just one
runtime.

How

Before prepending basePath, check whether the matcher regex (without basePath)
matches /. If it does, after building the final regex also accept the bare
basePath root by OR-ing in an anchored alternative:

^(?:/docs[\/#\?]?$|<original-body>)

The check is gated on basePath being set and the matcher not being the literal
/ root (which already has its own handling). Specific matchers like
/dashboard and param matchers like /:id do not match /, so they are left
unchanged and do not start matching the basePath root.

Tests

Extended get-page-static-info.test.ts with three cases:

• catch-all matcher with basePath: '/docs': /docs, /docs/, /docs/foo
  match; /docs/api/x, /docsfoo, /foo do not.
• static /dashboard: /docs/dashboard matches; /docs and /docs/ do not.
• param /:id: /docs/apple matches; /docs does not.

All nine tests in the file pass (six existing plus the three new ones):

pnpm jest packages/next/src/build/analysis/get-page-static-info.test.ts
Tests: 9 passed, 9 total

prettier and eslint were run on both changed files.

Signed commits

The commit is signed (SSH) and shows as verified, so it meets the protected
branch requirement.

[vercel]

The bare-basePath-root middleware matcher restoration omits transport-route forms, so the root RSC/data request (e.g. /docs.rsc) does not match even though the no-basePath analog /.rsc does.