Fix PathAliases auto-generated rules matching nested directories incorrectly - #2120

Summary

When using relative_files=true, the auto-generated path mapping rules used a regex pattern ^(.*[\\/])?{dir1}[\\/] that could match directory names anywhere in a path, not just from the start. This caused paths like pkg/features/templates.py to incorrectly match a rule generated for features/templates.py, resulting in lost coverage data during combine operations.

The Problem

For example, with two files:

• features/templates.py
• pkg/features/templates.py

The auto-generated rule for features/ would use regex ^(.*[\\/])?features[\\/], which incorrectly matches pkg/features/templates.py because (.*[\\/])? allows any prefix.

The Fix

Changed the auto-generated regex from:

^(.*[\\/])?{dir1}[\\/]  (matches anywhere)

to:

^{dir1}[\\/]            (anchored to start)

This ensures that nested directories with the same name as a top-level directory are not incorrectly collapsed during coverage combine.

Testing

• Added a new test test_nested_directories_same_name that verifies the fix
• All existing PathAliases tests pass (45 tests)
• All test_files.py tests pass (205 tests)

Fixes #2072