Non-extendable block built

[joshua-kim]

Seeing some WARN logs in datadog recently that indicate that the X-Chain is building non-extendable blocks. This is not expected behavior so this requires RCA + a fix.

{
                 ...
		"message": "block that was just built is not extendable",
		"attributes": {
			"caller": "snowman/engine.go:684",
			"level": "warn",
			"logger": "X Chain",
			"timestamp": "2025-05-12T11:47:12.011Z"
		}
	}
}