fix: Settings ignores an endpoint check once the same "Save" is tapped again (closes #698)

Each RPC and Blockscout "Save" in Settings now counts its taps. A check
that answers after a newer tap of the same "Save" stores nothing and
leaves the flash message alone. Before this change, the older check's
error or refusal replaced the newer tap's "Saved.".

The save also checks for a newer tap at two points. A tap made while the
save waits for the lock means nothing is stored. A tap made during the
write means no flash message is shown.

Model: opus-5-5

tests/settingsEndpointSave.test.js

@@ -330,3 +330,64 @@ test.each([
         expect(page.text("flash-msg")).toBe("Saved.");
     },
 );
+
+// The user taps "Save", and the check passes. The save then begins its storage
+// write, and this test keeps the write waiting. The user taps "Save" again, and
+// the second check cannot reach the endpoint. When the write ends, the flash
+// message still reads "Could not reach endpoint.". The write stores the URL
+// anyway, because the write began before the second tap.
+test.each([
+    { button: "btn-save-rpc", input: "settings-rpc", field: "rpcUrl" },
+    {
+        button: "btn-save-blockscout",
+        input: "settings-blockscout",
+        field: "blockscoutUrl",
+    },
+])(
+    "$button keeps the second tap's message when tapped again during the write",
+    async ({ button, input, field }) => {
+        const page = await bootPopup(unversionedValidProfile());
+        await page.click("btn-settings");
+        const checks = [];
+        globalThis.fetch.mockImplementation(
+            () =>
+                new Promise((resolve, reject) => {
+                    checks.push({ resolve, reject });
+                }),
+        );
+        const typedUrl = "https://typed.example.invalid";
+
+        page.node(input).value = typedUrl;
+        const firstTap = page.node(button).listeners.click.map((fn) => fn());
+        await page.settle();
+        const { set } = page.storage.local;
+        const write = set.getMockImplementation();
+        let writeBegan = false;
+        let releaseWrite;
+        const writeHeld = new Promise((resolve) => {
+            releaseWrite = resolve;
+        });
+        set.mockImplementationOnce(async (items) => {
+            writeBegan = true;
+            await writeHeld;
+            await write(items);
+        });
+        checks[0].resolve(PASSING_ANSWER);
+        await page.settle();
+        expect(writeBegan).toBe(true);
+        const secondTap = page.node(button).listeners.click.map((fn) => fn());
+        await page.settle();
+        // fetch rejects when it cannot reach the endpoint.
+        checks[1].reject(new TypeError("Failed to fetch"));
+        await Promise.all(secondTap);
+        await page.settle();
+        expect(page.text("flash-msg")).toBe("Could not reach endpoint.");
+        releaseWrite();
+        await Promise.all(firstTap);
+        await page.settle();
+
+        expect(checks).toHaveLength(2);
+        expect(page.storage.read("autistmask")[field]).toBe(typedUrl);
+        expect(page.text("flash-msg")).toBe("Could not reach endpoint.");
+    },
+);