Fix image path regex to handle edge cases and validate extensions

[Copilot]

Description

Addresses feedback from PR #3000 review comment #discussion_r2594633088 regarding the convertImagePathsToGitHub regex implementation.

Changes

• Consolidated regex patterns: Replaced three separate regex patterns with a single robust pattern that handles all path types (relative, absolute, direct)
• File extension validation: Only converts paths ending with valid image extensions (png, jpg, jpeg, gif, svg, webp, bmp, tiff, ico)
• Edge case handling:
  • Escaped brackets in alt text: ![Alt \[text\]](images/file.png)
  • Parentheses in paths: ![Alt](images/diagram(v2).png)
  • Title attributes: ![Alt](images/file.png "title")
• Comprehensive test coverage: Added 6 new tests covering all edge cases and validation scenarios
• Documentation: Added detailed inline comments explaining each regex component

Checklist

• Commits have been rebased.
• Linting has been run against the changed file(s).
• The PR adheres to the writing style guide and contribution guide.

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[coderabbitai]

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[Copilot]

Just as a heads up, I was blocked by some firewall rules while working on your feedback. Expand below for details.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• unpkg.com
  • Triggering command: /usr/local/bin/node /usr/local/bin/node scripts/postinstall.js (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

[sacOO7]

Separate PR is not needed, but it's good for reference 👍