bug: `return` instead of `continue` in signer message loop silently drops valid messages

[huth-stacks]

Bug Description

In stacks-signer/src/v0/signer.rs, the handle_event_match function processes batches of signer messages in a for loop. When a message from an unknown signer is encountered, the code executes return instead of continue, which exits the entire function — silently dropping all remaining valid signer messages in the batch.

File: stacks-signer/src/v0/signer.rs:523-541

// try and gather signatures
for (signer_public_key, message) in messages {
    let signer_address = StacksAddress::p2pkh(self.mainnet, signer_public_key);
    if !self.is_valid_signer(&signer_address) {
        debug!("{self}: Received a message from an unknown signer. Ignoring...";
            "signer_public_key" => ?signer_public_key,
            "signer_address" => %signer_address,
            "message" => ?message,
        );
        return;  // BUG: exits entire function, should be `continue`
    }
    match message {
        SignerMessage::BlockResponse(block_response) => self.handle_block_response(
            stacks_client,
            block_response,
            sortition_state,
        ),
        SignerMessage::StateMachineUpdate(update) => self
            .handle_state_machine_update(signer_public_key, update, received_time),
        // ...
    }
}

Impact

If any single message from an unknown signer appears anywhere in a batch, all subsequent valid messages (block responses, state machine updates, pre-commits) from legitimate signers are silently lost. This could:

• Delay or prevent blocks from reaching signer consensus
• Cause network stalls during periods of high signer activity
• Create hard-to-diagnose consensus failures since the message loss is completely silent (only a debug! log)

The bug is ordering-dependent: a batch [valid, valid, unknown, valid, valid] loses the last two valid messages.

Recommended Fix

Change return; to continue; on line 531. This skips only the unknown signer's message while allowing the loop to process all remaining valid messages.

-        return;
+        continue;

Suggested Test

Create a test that sends a batch of SignerEvent::SignerMessages where the first message is from an unknown signer and the second is from a valid signer with a StateMachineUpdate. After processing, assert that the valid signer's update was persisted to the signer DB. This test fails on master and passes with the fix.

---

Agent Prompt

Fix the signer message batch processing bug in stacks-signer/src/v0/signer.rs.

In the handle_event_match function, the SignerEvent::SignerMessages arm iterates
over messages in a for loop (line 523). When an unknown signer is encountered at
line 531, `return;` exits the entire function instead of `continue;`, dropping all
remaining valid messages in the batch.

Fix:
1. Change `return;` to `continue;` on line 531
2. Add a regression test that:
   - Creates a Signer with a known valid signer address
   - Constructs a batch with an unknown signer message first, then a valid
     signer's StateMachineUpdate second
   - Calls handle_event_match
   - Asserts that the valid signer's StateMachineUpdate was stored in signer_db
     via get_signer_state_machine_updates()
3. Run `cargo test -p stacks-signer` to verify