Fix input validation for directories and handle unreachable function in JSON checks

[aayushkdev]

Fixes #3592

When the --from-json option is used, it currently allows directories as input, resulting in a crash. This issue was initially addressed in PR #3609, but the fix did not fully resolve the problem. As a result, the tool continues to crash when directories are passed to --from-json.
This PR aims to resolve the issue by adding validation checks for directories.

It also fixes unreachable if statements in the validate_input_path function, ensuring more accurate validation of input files. Additionally, it ensures that files without the .json extension or those containing invalid JSON are correctly flagged, improving error handling and overall input validation.

Tasks

• Reviewed contribution guidelines
• PR is descriptively titled 📑 and links the original issue above 🔗
• Tests pass -- look for a green checkbox ✔️ a few minutes after opening your PR
  Run tests locally to check for errors.
• Commits are in uniquely-named feature branch and has no merge conflicts 📁
• Updated documentation pages (if applicable)
• Updated CHANGELOG.rst (if applicable)