fix(react-router): prevent incorrectly matching paths without slash separator

[nwleedev]

Problem

Optional dynamic segments (:param?) incorrectly match paths that extend the base path without a separator. For example:

matchPath('/test_route/:part?', '/test_route_more')
// Expected: null
// Actual: { params: { part: '_more' } }

The current regex pattern /?([^\/]+)? makes the slash and capture group independently optional.
It allows /test_route_more to match as /test_route + _more without a preceding slash.

Changes

The regex pattern to match paths

• Groups the slash with the capture
• Ensures slash and parameter appear together or not at all
• Uses * instead of + to allow empty captures for backward compatibility
  (e.g., /sitemap/:lang?.xml matching /sitemap/.xml)

Impact

• No breaking changes
  • Only fixes incorrect matching behavior
• Maintains backward compatibility with existing patterns

Fixes #14640

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