net: reject oversized outbound messages #35888

Cover the accepted 12-byte type and 4 MB payload boundaries for both outbound transports.
V2 accepts a 13-byte type when a non-empty payload keeps the copy in bounds.
The `connman` fuzzer stops at 12 bytes, and both transports accept a 4,000,001-byte payload while consuming the input message.

Extend the existing two-node `sendmsgtopeer` test to confirm the oversized payload is sent and makes the peer disconnect.

`Transport::SetMessageToSend()` encodes message types in a fixed 12-byte wire field.
V1 aborts on a longer name, while V2 can overwrite the first payload byte or write past its encoding buffer.

Reject oversized names before either transport encodes them and before `CConnman` queues them.
Verify both transports reject the message unchanged, and let the `connman` fuzzer exercise the first oversized length.

Normal outbound messages use fixed `NetMsgType` values, and `sendmsgtopeer` already rejects names longer than 12 bytes.

`ConnmanTestMsg::ReceiveMsgFrom()` asserted whenever the transport declined a message.
Return false for messages outside the wire limits so fuzz targets can exercise transport rejection without aborting.
Retain the assertion for every other queue failure.

Incoming transports reject payloads larger than 4 MB, while outbound transports accepted them.
The testing-only sendmsgtopeer RPC could send a 4,000,001-byte payload that made the peer disconnect.

Use one predicate to apply the payload limit before messages are queued or encoded, and log rejections from the void `PushMessage()` interface.
Return an RPC error from sendmsgtopeer.

The existing characterization now verifies that both transports reject the input unchanged, and the OSS-Fuzz input from #35880 passes without a harness restriction.