fix: align inbound and outbound packet size checks and drop oversize messages (#313)

[abhiramaab]

Purpose

Resolves #313 by aligning inbound Netty decoder limits with MQTT variable-length integer framing and adding outbound packet-size enforcement in MQTTSessionHandler.

Changes

1. Varint-Aware Inbound Limit (ChannelAttrs.java)

   • Accurately calculates maxRemainingLength(maxPacketSize) by accounting for the fixed header byte and the variable-length integer overhead.
   • Handles the 1–4 byte variable-length integer based on the remaining-length threshold instead of performing a raw subtraction.

2. Outbound Packet Size Enforcement (MQTTSessionHandler.java)

   • Enforces the client-negotiated/tenant maxPacketSize in publishConfirmable().
   • Drops packets that exceed the maximum size with DropReason.ResourceExhausted.
   • Emits OversizePacketDropped protocol events for dropped packets.
   • Confirms the message promise cleanly to prevent resource leaks.

3. Protocol Helper Support (IMQTTProtocolHelper.java)

   • Implements maxPacketSize() in both MQTT3ProtocolHelper and MQTT5ProtocolHelper.
   • Respects the MQTT 5 Maximum Packet Size connection property while applying the tenant-level limit.

4. Unit Tests (ChannelAttrsTest.java)

   • Added exhaustive boundary tests covering exact-fit and variable-length integer crossover points:
     • 127 / 128
     • 16,383 / 16,384
     • 2,097,151 / 2,097,152

Verification

• ChannelAttrsTest: Passed (2/2)
• TransientSessionHandlerTest & PersistentSessionHandlerTest: Passed (74/74)
• Checkstyle: 0 violations

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