fix(code-runtime-python): skip relative PATH entries in pythonBin resolution; pin the sealing-threshold rejection

The review's remaining code items:
- resolvePythonBin now skips RELATIVE PATH segments (a bare 'bin' or '.'): the
  returned candidate must be absolute, because spawn() resolves a relative
  pythonBin against the host CWD, outside the seam contract.
- A deterministic-ish regression pins the sealing-threshold corner: 64 MiB of
  4 KiB (<= PIPE_BUF, atomic) newline-free writes plus 12289 more A's before
  the first newline make the first frame exceed FRAME_PARSE_CAP_BYTES; the
  newline-bearing chunk reaches the first-frame check (sealing is the ELSE
  half of the newline branch), so the run reports worker-exit with the
  protocol-frame-exceeded message.

packages/code-runtime/code-runtime-python/src/index.ts

@@ -410,11 +410,13 @@ function resolvePythonBin(bin: string): string {
   /* v8 ignore next -- PATH is set in every environment the runtime boots in; the guard is defensive. */
   if (path === undefined) return bin
   for (const dir of path.split(delimiter)) {
-    // An empty PATH segment (a `::`, implicitly CWD on POSIX) is skipped so a
-    // basename never resolves against the working directory; normal PATHs
-    // carry no empty segment.
-    /* v8 ignore next -- normal PATHs carry no empty segment. */
-    if (dir === '') continue
+    // An empty PATH segment (a `::`, implicitly CWD on POSIX) and a RELATIVE
+    // segment (`bin` or `.`) are skipped: a basename must never resolve against
+    // the working directory, and the returned candidate must be an absolute
+    // path — spawn() resolves a relative pythonBin against the host CWD, which
+    // is outside the seam contract.
+    /* v8 ignore next -- normal PATHs carry no empty or relative segment. */
+    if (dir === '' || !isAbsolute(dir)) continue
     const candidate = join(dir, bin)
     try {
       accessSync(candidate, fsConstants.X_OK)