Fix subdirectory path support for starter_code files

[Akshita-2307]

Summary

• Allow serving files from subdirectories (e.g. starter_code/survey_form/index.html)
• Maintain path traversal protection by checking full_path starts with STARTER_CODE_DIR
• Use os.path.isfile() instead of os.path.exists() to avoid directories (fixes [Bug]: Project 8 (Password Strength Checker) returns 404 on starter code — file missing from repo #504)

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[Akshita-2307]

Program: GSSoC

[Akshita-2307]

Hi maintainers! This PR fixes subdirectory path support for starter_code files so challenges in nested directories can find their starter code correctly. Would appreciate a review. Thank you!

[komalharshita]

Thank you for the contribution.

I reviewed the changes and verified that the previous implementation discarded subdirectory information by using only the basename of the configured starter file path. This caused starter files stored in nested folders to fail resolution.

The updated implementation preserves relative subdirectory paths while still restricting file access to the configured STARTER_CODE_DIR, which addresses the reported issue without changing unrelated behavior.

The approach is focused, solves a real backend bug, and maintains the existing safety checks around file resolution.

Please rebase the branch and resolve the current merge conflicts. Once that is done, this looks ready to merge.