refactor: use native str.removesuffix instead of custom helper

[Ashutoshx7]

Description

This PR removes the redundant remove_suffix helper function from helpers.py and replaces its single usage in _app.py with Python's built-in str.removesuffix() method (available since Python 3.9).

Since the project requires Python ≥3.10, this is safe to use and reduces code maintenance burden.

Related Tickets & Documents

N/A - Code cleanup/modernization

What type of PR is this? (check all applicable)

• Refactor
• Feature
• Bug Fix
• Optimization
• Documentation Update

Steps to QA

1. Start the server with a custom base_url setting ending with /
2. Verify static files are served correctly (the trailing slash is stripped as expected)
3. Run existing tests to ensure no regressions

Added/updated tests?

• Yes
• No, and this is why: The native Python method str.removesuffix() is part of the standard library and is well-tested. The refactor is a 1:1 behavioral replacement verified through manual testing.
• I need help with writing tests

Added/updated documentations?

• Yes
• No, and this is why: This is an internal refactor with no user-facing changes.
• I need help with writing docs

Checklist

• I have added relevant notes to the CHANGELOG.md file (See https://keepachangelog.com/)