context: return preBlock from (*Context).PreBlock()

From the user's PoW PreBlock should be returned from this method, not
PreHeader, otherwise the behaviour is misleading and causes bugs.

Signed-off-by: Anna Shaleva <shaleva.ann@nspcc.ru>

CHANGELOG.md

@@ -16,6 +16,7 @@ Bugs fixed:
  * context-bound PreBlock and PreHeader are not reset properly (#127)   
  * PreHeader is constructed instead of PreBlock to create PreCommit message (#128)
  * enable anti-MEV extension with respect to the current block index (#132)
+ * (*Context).PreBlock() method returns PreHeader instead of PreBlock (#133)
 
 ## [0.3.0] (01 August 2024)
 

context.go

@@ -214,8 +214,11 @@ func (c *Context[H]) MoreThanFNodesCommittedOrLost() bool {
 	return c.CountCommitted()+c.CountFailed() > c.F()
 }
 
+// PreBlock returns current PreBlock from context. May be nil in case if no
+// PreBlock is constructed yet (even if PreHeader is already constructed).
+// External changes in the PreBlock will be seen by dBFT.
 func (c *Context[H]) PreBlock() PreBlock[H] {
-	return c.preHeader // without transactions
+	return c.preBlock
 }
 
 func (c *Context[H]) reset(view byte, ts uint64) {