Improve branch name handling and remove unused code

[sentry-autofix]

👋 Hi there! This PR was automatically generated by Autofix 🤖

This fix was triggered by Rohan Agarwal

Fixes SEER-J5

• Refactor _create_branch to include validation and handle edge cases like trailing slashes and empty branch names.
• Improve branch name sanitization in sanitize_branch_name to remove trailing slashes and handle empty segments.
• Add validation to create_branch_from_changes to ensure a valid branch name is generated, falling back to a random name if necessary.
• Remove process_one_file_for_git_commit as it is no longer used.

If you have any questions or feedback for the Sentry team about this fix, please email autofix@sentry.io with the Run ID: 5770.