fix: estimate a request even when the site sends its gas limit (closes #554)

ethers asks the node for a gas estimate only when the request has no gas
limit, and that estimate is the wallet's only check that a transaction
does not revert. Since a site's `gas` became the gas limit, a reverting
request carrying one opened the approval window and could be signed and
mined to fail. Such a request is now estimated first, with the site's
limit and under the same timeout; a failure goes back to the site with no
window, and the site's gas limit is still the one shown and signed. A
limit too low for the call now fails the same way.

Model: opus-5-5

README.md

@@ -2100,8 +2100,10 @@ view would leave a wallet one click from deletion.
   artifact can be compared with it field for field. A nonce the site supplies is
   ignored: the nonce is always the account's next nonce from the node, so a site
   cannot replace one of the user's pending transactions or leave this one stuck
-  behind a gap. A gas limit the site supplies, as `gas` or `gasLimit`, is used
-  in place of the node's estimate; a request giving both with different values
+  behind a gap. A gas limit the site supplies, as `gas` or `gasLimit`, is the
+  one shown and signed in place of the node's estimate, but the node still
+  estimates the transaction with that limit, as the check that it neither
+  reverts nor runs out of gas; a request that gives both with different values
   is refused. A request that cannot be populated — unreachable node, reverting
   gas estimate — opens no window and is failed back to the site. Only one
   transaction approval exists at a time: populating fixes the nonce, so a second