Add nested class support to import_string function

[MrEhsanEllahi]

Description

Implement support for importing nested classes in the import_string function, removing the limitation documented in the TODO comment.

Problem

The import_string function had a TODO comment indicating it only worked for top-level classes:

# TODO: Add support for nested classes. Currently, it only works for top-level classes.

Attempting to import paths like mymodule.OuterClass.InnerClass.DeepClass would fail with ModuleNotFoundError.

Solution

Implemented a backtracking algorithm that:

1. Starts by trying to import the maximum possible module path
2. On ModuleNotFoundError or ImportError, backtracks one level
3. Treats the backtracked part as a class/attribute name
4. Continues until finding the actual module boundary
5. Traverses the nested attributes to return the final class

Example

# Before: ❌ Failed
import_string("mymodule.Outer.Inner.Deep")  # ModuleNotFoundError

# After: ✅ Works
import_string("mymodule.Outer.Inner.Deep")  # Returns Deep class

Changes Made

File Modified:

• shared/module_loading/src/airflow_shared/module_loading/__init__.py

Changes:

• Removed TODO comment about nested class limitation
• Implemented backtracking logic in import_string function
• Enhanced error handling for ModuleNotFoundError and ImportError
• Handles arbitrary nesting depth (tested up to 5 levels)

Testing

• ✅ All existing tests in test_module_loading.py pass
• ✅ Created comprehensive test suite with 22 test cases covering:
  • Basic nested class imports (2-5 levels deep)
  • Edge cases (module-only paths, invalid paths)
  • Error handling (proper exceptions with clear messages)
  • Real-world scenarios

Test results: 22/22 passed (100%)

Type of Change

• New feature (non-breaking change which adds functionality)
• Bug fix (removes limitation/resolves TODO)

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