fix(flow): reset cancellation flag at the start of each execution

[XuQuanxin04]

Summary

LightFlow.cancel() sets self._cancelled = True, but that flag was only read in _execute() and never reset. Because run(), resume(), and rerun_step() all go through the same _execute() path on a reusable LightFlow instance, a single cancel() call permanently poisoned the instance — every subsequent run skipped all of its steps with "cancelled before execution", even when cancellation was requested after a run had already finished.

Reproduction

flow = LightFlow().step("a", agent=agent_a).step("b", agent=agent_b, depends_on=["a"])
flow.run("one")          # works: a, b succeed
flow.cancel()            # user cancels after run 1 completes
flow.run("two")          # BUG: both a and b are skipped; agent.run() never called

Fix

Reset self._cancelled = False at the start of _execute(). In-flight cancellation is unaffected — the flag is still re-checked before every step, so a cancel() from another thread during a run still skips the remaining steps of that run.

Compatibility

• Existing agent.run("hello") behavior is unchanged.
• Existing stream=True behavior is unchanged.
• In-flight cancel() behavior is preserved (added a test for it).

Tests

• python -m compileall -q LightAgent
• PYTHONPATH=. python -m pytest -q tests/test_v065_core.py tests/test_v070_tracing.py tests/test_memory_policy.py — 57 passed
• Added two tests to tests/test_lightflow.py:
  • test_lightflow_cancel_during_run_skips_remaining_steps_of_that_run — guards the existing in-flight cancel behavior.
  • test_lightflow_cancel_between_runs_does_not_poison_the_next_run — regression test for this bug.
• Full tests/test_lightflow.py: 16 passed.