Fix thread join bug

[Rabie45]

Purpose

Add a joined boolean field to the conn_t struct to accurately track
whether a thread has been properly joined. This prevents race conditions
and use-after-free bugs that could occur when threads were terminated
without proper synchronization.

The fix ensures that:

• Thread joined state is explicitly tracked per connection
• setup_thread() properly manages the joined flag
• Thread termination follows proper synchronization protocols
• Memory safety is maintained throughout thread lifecycle

This resolves the thread termination bugs described in issue #411
where improper thread state tracking could lead to crashes or
undefined behavior when connections were cleaned up.