Fixes redundant string to callable

[ooctipus]

Description

With resolvable string, the string to callable is now redundant, and many assumption built with string_to_callable at update class from cfg doesn't make sense anymore (Magically change string to type during update was an logic bug).

Fixes # (issue)

Type of change

• Bug fix (non-breaking change which fixes an issue)

Screenshots

Please attach before and after screenshots of the change if applicable.

Checklist

• I have read and understood the contribution guidelines
• I have run the pre-commit checks with ./isaaclab.sh --format
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• I have updated the changelog and the corresponding version in the extension's config/extension.toml file
• I have added my name to the CONTRIBUTORS.md or my name already exists there

[greptile-apps]

Greptile Summary

This PR cleans up redundant string-to-callable conversion logic in the config system. With the introduction of ResolvableString, callable strings are now kept lazy and only resolved on first use, eliminating the need for eager conversion during config updates.

• Removes redundant type check for ResolvableString in update_class_from_dict since it's already callable
• Adds optimization to _wrap_resolvable_strings to preserve container identity when no items need wrapping
• Updates tests to properly test lazy string resolution behavior
• Updates imports after string_to_callable was moved to string utils module

Confidence Score: 5/5

• This PR is safe to merge with minimal risk
• The changes are well-contained refactoring that removes redundant code and adds performance optimizations. The logic is sound, tests are properly updated, and the changes align with the lazy resolution pattern introduced by ResolvableString
• No files require special attention

Important Files Changed

Filename	Overview
source/isaaclab/isaaclab/utils/configclass.py	Adds optimization to _wrap_resolvable_strings to preserve container identity when no wrapping occurs
source/isaaclab/isaaclab/utils/dict.py	Removes redundant ResolvableString type check since ResolvableString is callable
source/isaaclab/test/utils/test_configclass.py	Updates test to use callable string format instead of direct function reference

_{Last reviewed commit: 437bbe3}

[greptile-apps]

_{5 files reviewed, no comments}

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[AntoineRichard]

We need that in ASAP :)