use granian env_files option instead of reload hook

[adhami3310]

No description provided.

[greptile-apps]

PR Summary

Refactors environment variable loading to use Granian's native env_files option instead of a custom reload hook, improving reliability and maintainability of .env file handling.

• Removed custom _reload_hook from reflex/utils/exec.py in favor of Granian's built-in environment file loading
• Added _paths_from_env_files and _paths_from_environment functions in reflex/environment.py for better separation of concerns
• Modified reflex/config.py to use new helper functions for parsing environment paths
• Switched from MPServer to Granian's Server class with native env_files support

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[codspeed-hq]

CodSpeed Performance Report

Merging #5512 will not alter performance

_{Comparing use-granian-env_files-option-instead-of-reload-hook (6d4e674) with main (e9c2e4b)}

Summary

✅ 8 untouched benchmarks