next-routing: remove unanchored has.value fallback

[fallintoplace]

Summary

This keeps @next/routing aligned with existing Next.js has.value matching semantics.

packages/next-routing/src/matchers.ts currently tries an anchored regex first and then falls back to an unanchored regex. That allows a rule like value: 'admin' to match not-admin, which is broader than the existing resolver.

This change removes the unanchored fallback and adds a regression test covering the not-admin case.

Related: #94109 was auto-closed by the issue template bot because it did not include a reproduction repository link.

Testing

• runtime verification of exact match vs substring non-match in packages/next-routing
• runtime verification that named capture regexes still resolve correctly
• added a regression test in packages/next-routing/src/__tests__/conditions.test.ts

Why

The goal here is compatibility. @next/routing is reimplementing subtle resolver behavior, so matcher semantics should stay as close as possible to the existing Next.js resolver.