fix: silently drop short GS4 query datagrams

[bejiihiu]

Fixes #1828.

GameSpyQueryHandler.channelRead0 reads magic bytes, type, and sessionId without first checking readableBytes(). A malformed UDP datagram shorter than 7 bytes throws IndexOutOfBoundsException on every packet, caught and logged at WARNING:

Error whilst handling query packet from

Fix:

• Added MINIMUM_MESSAGE_LENGTH (7) — a named constant with javadoc explaining the 2+1+4 byte layout and which handler branch reads the shortest valid message (QUERY_TYPE_HANDSHAKE).
• Silently drop any datagram shorter than MINIMUM_MESSAGE_LENGTH before touching any byte.
• Also guard the challenge-token read in the QUERY_TYPE_STAT branch (4 bytes after the 7-byte header) for defence-in-depth — a 7-byte datagram with type 0x00 would pass the header check but fail on the readInt() for the challenge token.

Test coverage:

• Short/empty datagrams are silently dropped (no exception)
• 7-byte STAT datagram without challenge token is silently dropped (no exception)

Validation:

• ./gradlew :velocity-proxy:check — checkstyle, spotless, and all tests pass