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

[LadyBluenotes]

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:
#4713