Abstract filesystem operations in doc_gen.py

[cpyle0819]

This PR completes the filesystem abstraction by updating doc_gen.py functions.

Changes Made

• Updated collect_snippets method to pass fs parameter to imported functions
• Ensures filesystem abstraction is used consistently throughout the DocGen class

Functions Updated

• collect_snippets method now passes fs=self.fs to:
  • collect_snippets function from snippets module
  • collect_snippet_files function from snippets module

Notes

• Other methods like process_metadata, find_and_process_metadata, and parse_config were already using filesystem abstraction correctly
• No additional tests needed as existing tests already use filesystem abstraction and our changes are minimal
• Code formatting verified with black

This completes the filesystem abstraction work across the core modules.