Improve YAML file path validation regex for all platforms

[veeceey]

Summary

• Improves YAML file path validation regex patterns for both Windows and Unix
• Previously, Windows regex was too permissive (matched any string ending in .yaml/.yml)
• Previously, Unix regex was too restrictive (only matched absolute paths starting with /)
• Better distinguishes between YAML file paths and raw YAML content strings

Changes

• Windows: Now properly validates paths with drive letters (C:), UNC paths (\), relative paths (.), and regular paths
• Unix: Now supports both absolute (/path) and relative (./path or path) file paths
• Removed TODO comment as regex is now comprehensive

Test plan

• Validated regex patterns match expected file path formats
• Existing functionality preserved for valid paths
• Better rejection of non-file-path strings

Generated with Claude Code

[veeceey]

Friendly ping - any chance someone could take a look at this when they get a chance? Happy to make any changes if needed.