fix: refuse an unsupported method with EIP-1193 code 4200 (closes #279)

handleRpc() answered a method it does not implement with
"Unsupported method: <method>" and no code, so a site probing for an
optional method could not tell "not implemented" from "the call failed"
and fall back. It now carries code 4200, which EIP-1193 defines for this
case; the message is unchanged. The provider already passes any code
through to the page.

The new test hands the real background's reply to the provider and
checks the page sees 4200.

Model: opus-5-5

src/content/inpage.js

@@ -31,12 +31,11 @@
     // an error instead of accepting the refusal.
     //
     // Whatever code arrived is passed through verbatim rather than being
-    // matched against a list: the extension emits codes such as 4001, 4100,
+    // matched against a list: the extension emits 4001, 4100, 4200 and 4902,
-    // 4200 and 4902, and a code this file has never heard of is still the
+    // and a code this file has never heard of is still the truth about what
-    // truth about what happened. An error reported with no code at all stays
+    // happened. An error reported with no code at all stays a plain Error —
-    // a plain Error — a ProviderRpcError whose `code` is undefined would
+    // a ProviderRpcError whose `code` is undefined would advertise a
-    // advertise a conformance it does not have. `message` is untouched in
+    // conformance it does not have. `message` is untouched in every case.
-    // every case.
     function toPageError(error) {
         const message = (error && error.message) || "Request failed";
         if (error && error.code !== undefined && error.code !== null) {