Fix CancelScope reuse not being detected on the asyncio backend

[jaideeppyne]

Changes

CancelScope.__enter__ on the asyncio backend guards against reuse with if self._active:, but __exit__ sets _active = False. So the guard only catches re-entering a scope that is still active, and entering a scope again after it has exited slips through silently, even though the guard's own message promises that each scope may only be used for a single with block. The Trio backend raises RuntimeError here.

_cancel_called also survives the first block, so reusing a cancelled scope silently cancels the body of the second block:

import anyio

async def main():
    scope = anyio.CancelScope()
    with scope:
        scope.cancel()

    with scope:
        await anyio.sleep(0)
        print("this never runs on asyncio")

anyio.run(main)                   # prints nothing, no error
anyio.run(main, backend="trio")   # RuntimeError: Each CancelScope may only be used for a single 'with' block

I track entry with a separate _has_been_entered flag that is never reset, which is how TaskGroup already guards its own re-entry in the same module. Both backends now raise the same RuntimeError.

The two new tests fail on asyncio, asyncio+uvloop and asyncio+eager without the patch, and already passed on Trio.

I found this with an asyncio/Trio differential harness. AI-assisted, and I ran and reviewed all of it myself.

Checklist

• You've added tests (in tests/) which would fail without your patch
• You've updated the documentation (in docs/), in case of behavior changes or new features
• You've added a new changelog entry (in docs/versionhistory.rst).

[fallenmi]

Verified exact head a58e99cc38bfafb3ed5d44506f578dd70223f80b and the identical current merge tree at 51585c78dce4fa83598156b79ca6ecc224ecc6c6.

The separate _has_been_entered flag correctly enforces one-shot use without overloading _active, which still tracks active-scope exit bookkeeping. This closes both ordinary post-exit reuse and the cancelled-scope path that silently cancelled the second block, and brings the asyncio backend in line with Trio. The two backend-parametrized regressions cover both paths, and the changelog entry is present.

All 18 exact-head check runs, the check suite and workflow, and both commit statuses are successful. I found no blocking issue. This was a source/API review; I did not rerun the suite locally.

Disclosure: this review was prepared with Codex assistance; I verified the exact refs, merge tree, source changes, policy, interactions, and live CI before submission.