Fix concurrent map writes in handleUpdates by adding mutex protection for inputGroupCalls

[ImmortalxAnimes]

The bot was crashing with fatal error: concurrent map writes during group call events. This happened because the inputGroupCalls map was being accessed from multiple goroutines at the same time. It was being written inside UpdateGroupCall while other parts of the code like OnRequestBroadcastTimestamp, OnRequestBroadcastPart, and OnUpgrade were reading from it. That race condition caused the random panics.

What I changed

• Added an inputGroupCallsMutex (sync.RWMutex) to the Context struct.
• Wrapped every access to inputGroupCalls with proper locking:
  • Writes (Lock/Unlock): UpdateGroupCall, getInputGroupCall
  • Reads (RLock/RUnlock): all callbacks, convertGroupCallId, Stop, joinPresentation

Fixes #36

Example

// Before: unsafe access
ctx.inputGroupCalls[chatID] = &tg.InputGroupCallObj{...}

// After: protected with mutex
ctx.inputGroupCallsMutex.Lock()
ctx.inputGroupCalls[chatID] = &tg.InputGroupCallObj{...}
ctx.inputGroupCallsMutex.Unlock()