fix(python): address review nits for loop runner, step timeout, and trailing newline

[Sunny (sunnylovestiramisu)]

Summary

Address reviewer nitpicks following PR #80:

1. Guard event loop closure in LoopRunner.stop():

   • loop.close() previously ran after self._thread.join(timeout=2.0) regardless of whether the background thread had finished. Calling close() on a still-running event loop raises RuntimeError.
   • Added if not self._thread.is_alive() and not self._loop.is_closed(): before closing.

2. Wire FleetConfig.step_timeout_s to SandboxHandle.exec:

   • step_timeout_s was previously validated in FleetConfig but never referenced.
   • Added default_timeout_s: float = 120.0 to SandboxHandle, populated from FleetConfig.step_timeout_s during SandboxFleet._acquire_one().
   • Updated SandboxHandle.exec and SandboxHandle.exec_async to default timeout_s: Optional[float] = None, falling back to self.default_timeout_s when omitted.

3. Code style / formatting:

   • Removed an extra blank line at the end of clients/python/src/ate_env/backend/__init__.py.

Testing

• Added unit test test_fleet_step_timeout_applies_to_handle in test_exec_semantics.py to verify that step_timeout_s set on FleetConfig propagates correctly to SandboxHandle.default_timeout_s.
• Ran full test suite:

  pytest clients/python/tests/
  # 119 passed, 29 skipped

[Tomer Glottmann (tomergee)]

Review at 80ecc61. Approve.

All three nits from the #80 second pass are done: loop.close() runs only when the thread has exited, step_timeout_s reaches SandboxHandle.default_timeout_s and is the default for exec and exec_async, the blank line is gone. 119 tests pass, ruff is clean, the branch merges clean with main at a82a87f and with #88.

Blocking: none. Before merge: none.

Nit

• handle.py:201-209 in #88 builds SandboxHandle without default_timeout_s, thus after both PRs land the async fleet's handles keep the 120 s default and ignore step_timeout_s. Whichever PR merges second passes default_timeout_s=self.config.step_timeout_s there too, with the same one-line test.