check: prohibit WatchOnly to send commits

Problem:

Inappropriate message in the WatchOnly node logs with NeoXAMEV extension
enabled:
```
2024-12-05T15:15:54.366+0300	INFO	[email protected]/dbft.go:545	received PreCommit	{"validator": 5}
2024-12-05T15:15:54.366+0300	DEBUG	[email protected]/check.go:90	can't send commit since self preCommit not yet sent
```

Based on these logs it may seem that WatchOnly is trying (or will try)
to send Commit. It must never happen, hence I dived into the library
code and discovered the condition that was added mostly for safety and
that was planned to be removed in future:
https://github.com/nspcc-dev/dbft/blob/7c005d93e4b47106b679e3cc55d182f8167e0e56/check.go#L82-L84

Ref. #118 (comment).

So the problem is in the invalid documentaiton and in improper log
message.

It should be noted that for non-WatchOnly CNs we also need to keep
and check PreCommitSent requirement because node's Commit must be
confirmed by previousely sent PreCommit.

Solution:

Strictly speaking, this commit almost doesn't change the dBFT behaviour
because WatchOnly node is not allowed to send Commit thanks to the
condition outlined above. But we'd better adjust the comment and don't
confuse the library users with inappropriate message.

Signed-off-by: Anna Shaleva <[email protected]>

CHANGELOG.md

@@ -11,6 +11,8 @@ Behaviour changes:
 Improvements:
 
 Bugs fixed:
+ * inappropriate log on attempt to construct Commit for anti-MEV enabled WatchOnly
+   (#139)
 
 ## [0.3.1] (29 November 2024)
 

check.go

@@ -79,15 +79,20 @@ func (d *DBFT[H]) checkPreCommit() {
 		d.preBlockProcessed = true
 	}
 
-	// Require PreCommit sent by self for reliability. This condition may be removed
-	// in the future.
+	// Require PreCommit sent by self for reliability. This condition must not be
+	// removed because:
+	// 1) we need to filter out WatchOnly nodes;
+	// 2) CNs that have not sent PreCommit must not skip this stage (although it's OK
+	//    from the DKG/TPKE side to build final Block based only on other CN's data).
 	if d.PreCommitSent() {
 		d.verifyCommitPayloadsAgainstHeader()
 		d.sendCommit()
 		d.changeTimer(d.SecondsPerBlock)
 		d.checkCommit()
 	} else {
-		d.Logger.Debug("can't send commit since self preCommit not yet sent")
+		if !d.Context.WatchOnly() {
+			d.Logger.Debug("can't send commit since self preCommit not yet sent")
+		}
 	}
 }