Support nested classes in import_string

[KushagraB424]

This PR adds support for importing nested classes using the import_string utility, resolving an existing TODO.

Currently, import_string strictly splits at the last dot and attempts to import the first part as a module. This immediately throws an ImportError if the dotted path points to a nested class inside another class (e.g., my_module.TopLevelClass.NestedClass).

The updated logic introduces a safe fallback mechanism. If the original split throws an ImportError, it iteratively searches backwards to find the longest valid base module path. Once the module is successfully imported, it recursively resolves the rest of the path using getattr().

This introduces no breaking changes to the default behavior. Unit tests have been added to verify that nested classes load properly.

closes: #71014

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[kaxil]

We can remove the TODO, I don't think we need to support that use-case unless we need it