fix(router-core): use configured search serializer for loader dependency keys

Problem

Custom stringifySearch is used for URL search state but not for loader dependency cache keys.

Routes using custom search serialization can therefore support values in search params that fail when used in loaderDeps.

Scope

• Use the configured search serializer when deriving loader dependency cache keys.
• Add a regression test using a custom serializer and a dependency value that JSON.stringify cannot handle.
• Verify loader dependency cache identity changes when the serialized dependency value changes.

Related Work

This continues the work started in #4713:
https://github.com/TanStack/router/pull/4713

Start in router-core's loader dependency cache-key derivation and its existing loader dependency tests; the issue does not name specific files or test paths. Add the regression coverage described in the scope, then verify that changing the serialized dependency changes cache identity and that the custom serializer handles the dependency value.