[bug]: OnionMessage does not implement SizeableMessage

[ziggie1984]

Pre-Submission Checklist

• I have searched the existing issues and believe this is a new bug.
• I am not asking a question about how to use lnd, but reporting a bug (otherwise open a discussion).

LND Version

LND Configuration

Backend Version

Backend Configuration

OS/Distribution

Bug Details & Steps to Reproduce

https://github.com/lightningnetwork/lnd/actions/runs/19344190869/job/55340167877?pr=10368

--- FAIL: TestSerializedSize (0.06s)
    serialized_size_test.go:17: [rapid] failed after 35 tests: (*T).FailNow() called
        To reproduce, specify -run="TestSerializedSize" -rapid.failfile="testdata/rapid/TestSerializedSize/TestSerializedSize-20251113203641-20695.fail" (or -rapid.seed=18082795565655846183)
        Failed test output:
    serialized_size_test.go:25: [rapid] draw msgType: 0x201
    test_utils.go:53: [rapid] draw privKeyBytes: []byte{0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0}
    test_message.go:837: [rapid] draw onionMessageDataLength: 0
    test_message.go:838: [rapid] draw onionMessageData: []byte{}
    serialized_size_test.go:48: 
        	Error Trace:	/home/runner/work/lnd/lnd/lnwire/serialized_size_test.go:48
        	            				/home/runner/go/pkg/mod/pgregory.net/rapid@v1.2.0/engine.go:371
        	            				/home/runner/go/pkg/mod/pgregory.net/rapid@v1.2.0/engine.go:234
        	            				/home/runner/go/pkg/mod/pgregory.net/rapid@v1.2.0/engine.go:120
        	            				/home/runner/work/lnd/lnd/lnwire/serialized_size_test.go:17
        	Error:      	Should be true
        	Test:       	TestSerializedSize
        	Messages:   	message type OnionMessage does not implement SizeableMessage
FAIL
FAIL	github.com/lightningnetwork/lnd/lnwire	31.421s
FAIL

Expected Behavior

Debug Information

Environment