fix(code-runtime-python): seal the open hold past MAX_PENDING_CHUNKS

The review's warning: each held open fragment is a distinct array slot plus
string object header (~30x overhead the byte cap cannot see), and a
budget-sized single-character open flood is honest-child reachable
(print('x', end='', flush=True) in a loop). With maxLogBytes near its ~67 MB
load ceiling that was up to ~2 GB of host auxiliary heap. The hold now seals
into one block past MAX_PENDING_CHUNKS, mirroring the fd-3 reader's blocks and
the stray capture's seal; the merge, truncateLogs, and the finish residual all
read sealed + current fragments, and a within-budget flood regression asserts
the merged entry is byte-identical.

packages/code-runtime/code-runtime-python/tests/runtime.spec.ts

@@ -1926,6 +1926,28 @@ describe('PythonCodeRuntime — programs and bindings', () => {
     expect(result.logs).toEqual(['xy'])
   }, 15_000)
 
+  it('seals the open hold past MAX_PENDING_CHUNKS without changing the merged entry', async () => {
+    // A budget-sized single-character open flood would otherwise accumulate
+    // thousands of fragment array slots (each a slot plus string header, ~30x
+    // overhead the byte cap cannot see). The hold seals into one block past
+    // MAX_PENDING_CHUNKS; the merged entry is byte-identical.
+    const { runtime } = await setup({ maxLogBytes: 65536 })
+    const result = await runtime.run({
+      program: [
+        'import os',
+        "os.write(3, b'{\"type\":\"log\",\"text\":\"x\",\"open\":true}\\n')",
+        // 3000 single-character open continuations (over MAX_PENDING_CHUNKS).
+        'for _ in range(3000):',
+        "    os.write(3, b'{\"type\":\"log\",\"text\":\"a\",\"open\":true}\\n')",
+        "os.write(3, b'{\"type\":\"log\",\"text\":\"y\"}\\n')",
+        'return "done"',
+      ].join('\n'),
+      bindings: [],
+    })
+    expect(result.error).toBeUndefined()
+    expect(result.logs).toEqual(['x' + 'a'.repeat(3000) + 'y'])
+  }, 15_000)
+
   it('bounds a forged open-frame flood against the log budget', async () => {
     // The open hold must be bounded by the ledger: without the exact-cost check
     // a forged open flood would grow the held fragment without touching