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!