fix(cli): stop the preflight cancel test reading a pid file mid-write (#4077)

The test polled a pid file written by `echo $$ > path`, whose shell
redirection creates (truncates) the file before the echo writes to
it. A read landing in that window parsed an empty string to 0, which
satisfied "not undefined" and exited the wait loop early, failing
the next assertion with "expected 0 to be greater than 0" — the
exact shape of the intermittent CI failure. The loop now only
accepts a parsed value that's a positive integer, reproduced with a
busy-wait harness that hit the empty-file race in most iterations
and confirmed clean after the fix.

packages/cli/src/browser/preflight.test.ts

@@ -88,10 +88,14 @@ describe("runEnvironmentChecks", () => {
         const deadline = Date.now() + 2_000;
         while (probePid === undefined && Date.now() < deadline) {
           try {
-            probePid = Number(readFileSync(pidPath, "utf8").trim());
+            // Shell `>` redirection creates (truncates) the file before `echo $$`
+            // writes to it — a read can land on that empty window and parse to 0.
+            const pid = Number(readFileSync(pidPath, "utf8").trim());
+            if (Number.isInteger(pid) && pid > 0) probePid = pid;
           } catch {
-            await new Promise((resolve) => setTimeout(resolve, 10));
+            // pid file not created yet
           }
+          if (probePid === undefined) await new Promise((resolve) => setTimeout(resolve, 10));
         }
         expect(probePid).toBeGreaterThan(0);