Fix: robustify file handling and resolve path creation bugs in utils

[kamilansri]

Description

This PR addresses several bugs and cleans up the utility functions handling file discovery and result saving for the text detection outputs.

Changes Made

• Extension Bug Fix: Added the missing dot to the .tiff check in list_files (previously 'tiff'), ensuring TIFF files are correctly identified.
• Performance & Readability: Converted the long if ext == ... or ext == ... chains into O(1) set lookups (in {'.jpg', '.png', ...}).
• Path Safety: Refactored manual string concatenation (dirname + "res_" + filename + '.txt') to use os.path.join to prevent path malformations if dirname is passed without a trailing slash.
• Robust Directory Creation: Replaced os.mkdir with os.makedirs(dirname, exist_ok=True). The previous implementation would crash if the parent directory didn't exist or if a race condition occurred in a multi-threaded environment.

Checklist

• Tested directory creation with nested, non-existent paths.
• Code follows project styling conventions.