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

tests/approvalTx.test.js

@@ -139,14 +139,20 @@ describe("prepareApprovalTx", () => {
     });
 
     // `gas` is the JSON-RPC name for the gas limit and the one sites send.
-    // The node would estimate 21000; the site's 200000 is what is shown and
-    // what is signed.
+    // The node is still asked for its estimate, with the site's gas limit,
+    // because the estimate is the check that the transaction does not revert.
+    // It estimates 21000; the site's 200000 is what is shown and what is
+    // signed.
     test("uses the gas limit a site sends as gas", async () => {
+        const estimateGas = jest.fn(async () => 21000n);
         const approved = await prepareApprovalTx(
-            providerWith(),
+            providerWith({ estimateGas }),
             signer.address,
             { ...TX_PARAMS, gas: "0x30d40" },
         );
+        expect(estimateGas).toHaveBeenCalledWith(
+            expect.objectContaining({ gasLimit: 200000n }),
+        );
         expect(approved.gasLimit).toBe("0x30d40");
         const signed = Transaction.from(
             await signer.signTransaction({ ...approved }),
@@ -297,6 +303,22 @@ describe("prepareApprovalTx", () => {
         expect(thrown.message).toMatch(/^[A-Z].*\.$/);
     });
 
+    test("fails back a reverting request that sends its own gas limit", async () => {
+        const reverting = providerWith({
+            estimateGas: async () => {
+                throw new Error("execution reverted: ERC20: transfer amount");
+            },
+        });
+        await expect(
+            prepareApprovalTx(reverting, signer.address, {
+                ...TX_PARAMS,
+                gas: "0x30d40",
+            }),
+        ).rejects.toThrow(
+            /^The transaction could not be prepared: execution reverted/,
+        );
+    });
+
     // Without a bound, an unreachable node leaves the page's promise pending
     // with nothing on screen to explain it.
     test("gives up on a node that never answers", async () => {