fix(dashboard): replace self-calling restart logic with standalone endpoint

[Akinator31]

This pull request refactors the bot start/stop logic in the API to centralize the implementation, simplify handler code, and remove unused internal token-based API calls. The main changes include extracting bot lifecycle management into reusable functions, updating handlers to use these new utilities, and cleaning up unused code and fields.

Bot lifecycle management refactor:

• Added start_bot and stop_bot utility functions in rustmail/src/api/utils/bot.rs, along with StartBotResponse and StopBotResponse enums to standardize responses.
• Updated handle_start_bot, handle_stop_bot, and handle_restart_bot handlers to use the new utility functions, simplifying their logic and improving maintainability.
• Exported the new bot utilities in rustmail/src/api/utils/mod.rs.

Removal of internal token logic and related code:

• Removed the internal_token field from BotState and its initialization, as well as related token generation code in rustmail/src/bot.rs and rustmail/src/types/bot.rs.
• Deleted the unused ping_internal utility and its module export.
• Simplified the auth_middleware by removing internal token checks and associated imports.

These changes make the bot API more modular and easier to maintain, while removing unused or redundant logic.