fix: honour a transaction response only for a transaction approval (closes #262)

The liveness fix this issue describes — settle 4001 on release when the window
a retry would use is gone — already landed with
#271. This completes the rest.

AUTISTMASK_TX_RESPONSE now refuses any approval that is not a transaction
approval, so a reject can no longer retire a sign or connection approval, and a
signed artifact never runs the broadcast path against one — which before only
failed closed by throwing deeper in. Tests pin the site-connection port's
approve, reject and disconnect paths against a transaction approval broadcasting
behind them: each is declined and the dApp still receives its broadcast result.

Model: opus-4-8

src/background/index.js

@@ -1344,6 +1344,15 @@ runtime.onMessage.addListener((msg, sender, sendResponse) => {
         const approval = pendingApprovals[msg.id];
         if (!approval) return false;
 
+        // This message signs and broadcasts a transaction, so it is honoured
+        // only for a transaction approval. A sign or connection approval
+        // carries no approvedTx, and reaching the broadcast path with one used
+        // to fail closed by throwing deeper in; refusing here keeps a future
+        // refactor from turning that incidental throw into a live path, and
+        // keeps a reject on this message from retiring an approval of another
+        // kind.
+        if (approval.type !== "tx") return false;
+
         // A reject arriving while an attempt holds the approval is refused,
         // not honoured: the attempt is on its way to broadcasting the
         // transaction, and resolving 4001 here would tell the page the request