fix(FsspecStore): close owned async filesystem on store.close()

[josh-ag2]

Summary

`FsspecStore.from_url()` and `from_mapper()` create their own async filesystem instance, but `Store.close()` never cleaned it up. This left the aiohttp `ClientSession` inside s3fs open until garbage collection, producing:

```
ResourceWarning: Unclosed client session
ResourceWarning: coroutine 'ClientCreatorContext.aexit' was never awaited
```

The test file already had a `# TODO: fix these warnings` comment and a module-level `pytestmark` filter suppressing these warnings — this PR resolves the underlying cause for the `close()`-triggered path.

Changes

`src/zarr/storage/_fsspec.py`

• Add `_close_fs(fs)` — async helper that calls `fs.set_session()` then `client.close()` for filesystems that expose `set_session()` (i.e. s3fs / aiobotocore); silently skips all others.
• Add `_owns_fs: bool` to `FsspecStore.init` (default `False`). Set `True` in `from_url()` unconditionally (zarr created the fs). Set `True` in `from_mapper()` only when `_make_async()` produced a new instance (sync→async conversion). Direct construction and `from_upath()` leave `_owns_fs=False` — the caller supplied the fs and remains responsible for its lifecycle.
• Override `close()` to call `zarr_sync(_close_fs(self.fs))` before `super().close()` when `_owns_fs is True`. Wrapped in `contextlib.suppress(Exception)` so it can never raise from a destructor context.

Tests

Eight new tests in `tests/test_store/test_fsspec.py`:

• `test_from_url_owns_filesystem` — `_owns_fs=True` after `from_url()`
• `test_from_url_close_releases_store` — `close()` completes without error and sets `_is_open=False`
• `test_direct_construction_does_not_own_filesystem` — `_owns_fs=False` when fs supplied directly
• `test_from_upath_does_not_own_filesystem` — `_owns_fs=False` for `from_upath()`
• `test_from_mapper_does_not_own_already_async_filesystem` — `_owns_fs=False` when mapper's fs is already async (identity case in `_make_async`)
• `test_from_mapper_owns_wrapped_sync_filesystem` — `_owns_fs=True` when sync fs is wrapped
• `test_close_fs_closes_s3_client` — unit test (mock): `_close_fs()` calls `set_session()` then `client.close()`
• `test_close_fs_no_op_for_fs_without_set_session` — unit test (mock): `_close_fs()` is a no-op for fs without `set_session`

All 105 existing + new tests pass; 6 skipped (version-gated).

Scope

This fix closes the S3 client session that is active at the time `store.close()` is called. It does not prevent warnings that may arise from sessions abandoned earlier — for example, s3fs with `cache_regions=True` internally refreshes and replaces its S3 client during I/O, discarding prior sessions before `close()` is ever invoked. Those intermediate sessions are an upstream s3fs issue addressed in fsspec/s3fs#1028, which fixes:

• stale `S3BucketRegionCache` not closed on refresh
• vacuous-truth stale-session check on an empty `_sessions` dict
• concurrent `asyncio.gather` callers each creating a separate creator and tearing down each other's in-flight sessions

Notes

• The `pytestmark` `ResourceWarning` filter is kept: it still applies to stores GC'd without an explicit `close()` call (e.g. `test_basic`) and to the residual aiobotocore `ClientCreatorContext.aexit` issue. The filter comment is updated to reflect what's now fixed vs what's still outstanding.
• `from_upath()` and direct `FsspecStore(fs=..., ...)` construction leave `_owns_fs=False` — the caller owns the fs in those cases, matching the existing ownership model.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.50%. Comparing base (c0e2afa) to head (f4d4517).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4003   +/-   ##
=======================================
  Coverage   93.49%   93.50%           
=======================================
  Files          88       88           
  Lines       11873    11889   +16     
=======================================
+ Hits        11101    11117   +16     
  Misses        772      772           

Files with missing lines	Coverage Δ
src/zarr/storage/_fsspec.py	92.06% <100.00%> (+0.73%)	⬆️

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