fix: resolve infinite loop in _find_config on Windows systems

[Julfried]

Issue #4967

Description of changes:
This PR fixes an infinite loop in _find_config that occurs on Windows systems. The issue was caused by using Path.match("/") to detect root directories, which never matches on Windows due to different path separators.

Changes made:

1. Replace platform-specific Path.match("/") with Path.anchor comparison
2. Implement proper root directory detection for both Windows and Unix systems
3. Add comprehensive cross-platform tests

Testing done:

• Verified fix on both Windows and Linux systems
• Added test_find_config_cross_platform to validate:
  • Basic path traversal
  • Nested directory structures
  • Root directory handling
  • Non-existent config scenarios
• Added test_find_config_path_separators to verify handling of:
  • Forward slashes
  • Backslashes
  • Mixed separators
• All existing tests pass without modification

Merge Checklist

General

• I have read the CONTRIBUTING doc
• I certify that the changes I am introducing will be backward compatible
• I used the commit message format described in CONTRIBUTING
• I have passed the region in to all S3 and STS clients (N/A for this change)
• I have updated necessary documentation (N/A - internal implementation change)

Tests

• I have added tests that prove my fix is effective
• I have added unit tests to ensure backward compatibility
• I have checked that my tests are not configured for a specific region or account
• I have used unique_name_from_base for resource names (N/A - unit tests only)
• No new dependencies added

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[Julfried]

Quick update: I was unsure about two checklist items and checked them with N/A, because I think they are not applicable for my changes:

1. "Passed region to S3/STS clients"
2. "Documentation updated"

Please let me know if there is anything I need to do, to get this merged. Thanks!