fix: block placement validation, handshake null safety, processQueue atomic polling, and error propagation

[GamingOP69]

Summary

Adds packet bounds validation for block placement, null guards in PlayerHandshakeEvent / HandshakeListener, atomic polling in processQueue, and prevents JVM Error suppression.

Problem & Root Cause

• Malformed PacketPlayInBlockPlace packets containing invalid face directions (not in 0..5, 255) or negative Y coordinates were passed directly into block shift calculations, causing server crashes.
• When proxy authentication plugins (such as TCPShield or BungeeGuard) forwarded empty or unexpected handshake headers, HandshakeListener threw NPE when reading socket addresses or profile properties.
• MinecraftServer polled processQueue using .remove() after checking !isEmpty(), which is not thread-safe. Additionally, catching raw Throwable suppressed JVM critical errors (such as OutOfMemoryError or StackOverflowError), leaving the server in an unrecoverable corrupted state.
• AntiCrash threw NPE on custom payload packets with null sub-channels.

Solution

• Added explicit bounds checks in PlayerConnection.a(PacketPlayInBlockPlace) for null block position, null direction, and negative Y coordinates.
• Added null safety checks in HandshakeListener and PlayerHandshakeEvent.
• Switched processQueue polling to poll() with null check, and narrowed the catch block to Exception so critical JVM Error instances propagate correctly.
• Added null validation in AntiCrash packet inspection.
• Added unit tests: BlockPlaceValidationTest, PlayerHandshakeEventTest, PlayerHandshakeEventRegressionTest, and ProcessQueueSafetyRegressionTest.