test: repeat block malleability test with relayable block over P2P

Adds a functional test that repeats the existing witness nonce size
malleability check using a block under MAX_BLOCK_WEIGHT so it can be
relayed over the P2P network, addressing the TODO in test_block_malleability.

Includes rejection check for 'bad-witness-nonce-size' and confirmation
that a corrected block is accepted.

test/functional/p2p_segwit.py

@@ -82,6 +82,7 @@ from test_framework.script_util import (
 from test_framework.test_framework import BitcoinTestFramework
 from test_framework.util import (
     assert_not_equal,
+    assert_greater_than_or_equal,
     assert_equal,
     assert_raises_rpc_error,
     ensure_for,
@@ -822,7 +823,6 @@ class SegWitTest(BitcoinTestFramework):
         assert block.get_weight() > MAX_BLOCK_WEIGHT
 
         # We can't send over the p2p network, because this is too big to relay
-        # TODO: repeat this test with a block that can be relayed
         assert_equal('bad-witness-nonce-size', self.nodes[0].submitblock(block.serialize().hex()))
 
         assert_not_equal(self.nodes[0].getbestblockhash(), block.hash_hex)
@@ -833,6 +833,36 @@ class SegWitTest(BitcoinTestFramework):
 
         assert self.nodes[0].getbestblockhash() == block.hash_hex
 
+        # Build a relayable-but-invalid block
+        relayable_block = self.build_next_block()
+        add_witness_commitment(relayable_block)
+        relayable_block.solve()
+
+        # Append extra witness data to the coinbase input that triggers the
+        # same validation rejection but keeps the block under the weight limit
+        # so it can be sent via P2P.
+        relayable_block.vtx[0].wit.vtxinwit[0].scriptWitness.stack.append(b'a' * 100_000)
+
+        # Ensure it's relayable by weight
+        assert_greater_than_or_equal(MAX_BLOCK_WEIGHT, relayable_block.get_weight())
+
+        # Send over P2P and expect rejection for the same reason
+        test_witness_block(self.nodes[0], self.test_node, relayable_block,
+                           accepted=False, reason='bad-witness-nonce-size')
+
+        # Node should still be on the previous tip
+        assert_not_equal(self.nodes[0].getbestblockhash(), relayable_block.hash_hex)
+
+        # Now fix the block by removing the extra witness data
+        relayable_block.vtx[0].wit.vtxinwit[0].scriptWitness.stack.pop()
+
+        # Confirm the block is still relayable by weight
+        assert relayable_block.get_weight() <= MAX_BLOCK_WEIGHT
+
+        # Send the corrected block and expect acceptance
+        test_witness_block(self.nodes[0], self.test_node, relayable_block, accepted=True)
+        assert_equal(self.nodes[0].getbestblockhash(), relayable_block.hash_hex)
+
         # Now make sure that malleating the witness reserved value doesn't
         # result in a block permanently marked bad.
         block = self.build_next_block()