fix: carry EIP-1193 error codes through to the page (closes #274)

The provider rebuilt every rejection as a bare Error carrying only a message,
so a dApp checking err.code === 4001 saw undefined and could not tell a user's
deliberate refusal from a failure. Well-behaved sites therefore showed an error
or retried instead of accepting the refusal. The code was produced correctly
and did cross the extension boundary; it was lost in the last hop.

Rejections now reach the page as a ProviderRpcError carrying code, and data
where present. The code is passed through verbatim rather than matched against
a whitelist, so a code added upstream later needs no change here. An error that
genuinely has no code stays a plain Error with no code property at all, rather
than advertising code: undefined -- 'code' in err is what a careful dApp asks.

Messages are unchanged for every path, verified byte-for-byte against the
previous provider across every background error shape.

The end-to-end assertion that printed the observed code now requires it.

tests/e2e/run.js

@@ -1591,15 +1591,14 @@ async function lastResponseError(page) {
 }
 
 // A rejected prompt, asserted at both ends: the page's promise rejected
-// rather than hanging or resolving, and the response that crossed the
-// boundary carried EIP-1193 code 4001.
+// rather than hanging or resolving, and EIP-1193 code 4001 is present both
+// on the wire and on the Error the calling page catches.
 //
-// The code is asserted on the wire because that is the only place it
-// survives. src/content/inpage.js rebuilds the rejection as `new
-// Error(error.message)`, so the Error the calling page catches carries the
-// message and no code. That is reported rather than asserted either way —
-// locking in the current behaviour would make the gap permanent, and
-// asserting the code on the Error would fail today.
+// Both ends matter because they used to disagree. The code crossed the
+// boundary correctly and src/content/inpage.js then threw it away, rebuilding
+// every rejection as `new Error(error.message)` — so a dApp branching on
+// `err.code === 4001` saw undefined and could not tell a refusal from a
+// failure (#274). Asserting only the wire would leave that gap invisible.
 async function assertUserRejection(page, key, label) {
     const outcome = await settleRequest(page, key);
     assert(
@@ -1621,15 +1620,32 @@ async function assertUserRejection(page, key, label) {
             " did not carry EIP-1193 code 4001 across the boundary: " +
             JSON.stringify(error),
     );
+    assert(
+        outcome.hasCode,
+        label +
+            " reached the page as an error with no code property at all, so a " +
+            "dApp cannot tell the user's refusal from a failure: " +
+            JSON.stringify(outcome),
+    );
+    assert(
+        outcome.code === 4001,
+        label +
+            " reached the page with code " +
+            JSON.stringify(outcome.code) +
+            " rather than EIP-1193 4001",
+    );
+    assert(
+        outcome.name === "ProviderRpcError",
+        label +
+            " reached the page as " +
+            JSON.stringify(outcome.name) +
+            " rather than an EIP-1193 ProviderRpcError",
+    );
     console.log(
         "# " +
             label +
-            ": boundary code=" +
-            error.code +
-            " page Error.code=" +
-            JSON.stringify(outcome.code) +
-            " page Error carries a code=" +
-            outcome.hasCode,
+            ": code 4001 on the wire and on the page's " +
+            outcome.name,
     );
     return outcome;
 }