Localize sandbox thread quota enforcement to sandbox imports

[seanwevans]

Motivation

• Prevent global mutation of threading.Thread.start which could affect non-sandbox host threads.
• Keep child-thread quota/accounting enforced for sandboxed code while ensuring host threads remain untouched.

Description

• Replace the global threading.Thread.start patch with a sandbox-scoped Thread subclass factory via _make_sandbox_thread_class that implements quota checks and accounting.
• Return a wrapped threading module from _wrap_module(...) when sandboxed code imports threading, with Thread set to the sandbox-scoped class so only sandbox imports are affected.
• Remove the global patch/unpatch operations from the sandbox run loop so non-sandbox threads are never modified.
• Add a regression test test_sandbox_threading_patch_is_local_and_does_not_touch_global_start in tests/test_thread_extra.py to verify host threads run concurrently and that threading.Thread.start identity never changes globally.

Testing

• Ran pytest -q tests/test_thread_extra.py tests/test_thread_quota.py, all tests passed: 13 passed.
• The new regression test verifies no global behavior bleed while sandbox spawns threads and host threads continue using the original threading.Thread.start.

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Codex Task