Python: [Bug]: FileSystemAgentFileStore.delete can violate its result contract during concurrent deletion

@westey-m is already working on this.

Since Sep 17, 2026.

## Description

`AgentFileStore.delete()` documents `True` when deletion occurs and `False` when the file does not exist. `FileSystemAgentFileStore` currently performs its file check and deletion as separate operations. When two store instances share a root and delete the same path concurrently, both can pass the check and both report `True`, even though only one deletion occurs.

Against current `main` at `a3fc29dbd63a5223ece479b9b7eed53f50144999`, the exact reproduction below printed:

```text
attempt 2: [True, True]
```

For two concurrent same-path deletes within the same process, with no intervening write, one operation should return `True` and the other `False`. This does not request cross-process atomicity or delete/write transactional behavior.

I intend to submit a small fix that coordinates same-path deletion within the process, preserves the existing directory/non-file behavior, and maps only a `FileNotFoundError` deletion race to `False`.

## Code Sample

```python
import asyncio
from tempfile import TemporaryDirectory

from agent_framework import FileSystemAgentFileStore

async def main() -> None:
with TemporaryDirectory() as root:
first = FileSystemAgentFileStore(root)
second = FileSystemAgentFileStore(root)

for attempt in range(1, 1_001):
await first.write("shared.txt", "content")
try:
results = await asyncio.gather(
first.delete("shared.txt"),
second.delete("shared.txt"),
)
except FileNotFoundError as exc:
print(f"attempt {attempt}: {type(exc).__name__}: {exc}")
return

if sorted(results) != [False, True]:
print(f"attempt {attempt}: {results}")
return

print("no contract violation observed in 1000 attempts")

asyncio.run(main())
```

## Error Messages / Stack Traces

No exception was raised in the observed run. The exact output was:

```text
attempt 2: [True, True]
```

## Package Versions

`agent-framework-core`: source checkout of `main` at `a3fc29dbd63a5223ece479b9b7eed53f50144999` (workspace version `1.18.0`)

## Python Version

Python 3.12.12

## Additional Context

Observed on macOS/Darwin arm64. Issue #8255 and PR #8261 addressed analogous concurrent deletion behavior in `FileCheckpointStorage`; `FileSystemAgentFileStore` is a separate implementation and retains the race described here.