fix: Simplify session store and delete logic

[Unbreathable]

This MR does the following things:

• Simplifies session store and delete logic + gets rid of the global mutex
• Makes sure sessions are properly copied and the cache is only accessed where absolutely needed (to prevent messing up with mutexes)

[Copilot]

Pull request overview

This PR refactors the session management system to remove the global mutex (SessionCache struct with its global mutex) and simplify the session store and delete logic. The changes aim to improve concurrency by relying on sync.Map's built-in synchronization and per-SessionsList mutexes rather than a single global lock.

Changes:

• Removed the SessionCache wrapper struct with its global mutex, replacing it with direct use of sync.Map
• Refactored createSession to use LoadOrStore pattern instead of multiple Load operations with global locking
• Updated GetSessions to create defensive copies of session slices to prevent external modification
• Simplified SendEventToUser and GetConnections to reuse GetSessions method

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
neogate.go	Removes SessionCache struct and global mutex, replaces with direct sync.Map usage
sessions.go	Refactors session creation/deletion logic and adds defensive copying in GetSessions
send.go	Simplifies SendEventToUser by reusing GetSessions method, removes unnecessary error handling
connections.go	Simplifies GetConnections by delegating to GetSessions

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