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

TODO.md

@@ -45,6 +45,13 @@ but the review is broader than any of them.
 
 # Completed Steps
 
+- 2026-10-04: A method the wallet does not implement is refused with EIP-1193
+  code `4200` ([#279](https://git.eeqj.de/sneak/AutistMask/issues/279)). The
+  background's `Unsupported method: <method>` error carried no code, so a site
+  probing for an optional method could not tell "not implemented" from "the call
+  failed". The message is unchanged; the background's other errors with no code
+  are untouched.
+
 - 2026-10-04: Settings lists the sites connected without "Remember", and
   removing a site there disconnects it
   ([#406](https://git.eeqj.de/sneak/AutistMask/issues/406)). Such a connection