test: delete the unreachable seal() hook and guard non-object RPC bodies

Three review findings, no redesign.

seal() was installed after session.close(), so onLate could never fire:
once the context is destroyed no route handler and no console listener
exists to record anything. It was dead code that the attribution table,
run.js and harness.js all described as the final safety net. Deleted
rather than moved — take() already drains everything the collector holds
before teardown, so it covered nothing, and this harness must not ship a
mechanism it cannot demonstrate. The now-unreachable `late > 0` term in
the failure condition goes with it.

handleRpc() parsed `postData || "null"` and then dereferenced the result,
so a POST whose body Playwright reports as null — a bodyless request, or
any payload it cannot decode as UTF-8, e.g. sendBeacon with a Blob —
threw a TypeError inside the route handler and killed node mid-suite:
truncated TAP, no summary, no failure line. Non-object payloads now take
the same path as unparseable ones and are reported as unstubbed traffic,
which is the entire point of that branch.

README claimed unrecognised outbound requests are reported as failures,
unqualified, while observation in fact ends TRAILING_WATCH_MS after the
last test returns. The limit is now stated where it lands.

tests/e2e/network.js

@@ -131,6 +131,23 @@ function handleRpc(route, postData, report) {
     }
     // ethers batches by default, so the body may be an array.
     const batch = Array.isArray(payload) ? payload : [payload];
+
+    // Anything that is not a JSON-RPC object, or a batch of them, is not
+    // RPC at all and must be reported like any other unrecognised
+    // outbound traffic rather than dereferenced. request.postData()
+    // returns null both for a bodyless POST and for a body Playwright
+    // cannot decode as UTF-8 (sendBeacon with a Blob, or any binary
+    // payload), so this is not an empty-string special case: it rejects
+    // every non-object payload, exactly as the catch above rejects every
+    // unparseable one.
+    if (
+        payload === null ||
+        typeof payload !== "object" ||
+        !batch.every((req) => req !== null && typeof req === "object")
+    ) {
+        report("unstubbed request: POST " + route.request().url());
+        return route.abort();
+    }
     const replies = batch.map((req) => {
         const result = RPC_RESULTS[req.method];
         if (result === undefined) {