fix(storage): yield only direct children from FsspecStore.list_dir

[d-v-b]

AI PR that ensures that FspecStore.list_dir doesn't return garbage for http stores

🤖 AI text below 🤖

Closes #3575.

Problem

Over HTTP, fsspec builds a directory listing by scraping the links on an HTML index page. Some of those links aren't children of the directory, and FsspecStore.list_dir passed them through as keys:

• A link to the site root (href="/") is treated by fsspec as relative and joined onto the directory URL, producing the directory itself. list_dir stripped that down to "", and Group.getitem("") resolves to the group itself, so the group listed itself as a member named "". This is the bug reported in Zarr over https returns an empty string in keys() #3575.
• In-page anchors (href="#usage") became keys like #usage. Fetching metadata under such a key drops the fragment and returns the HTML page, so on the ECMWF dataset from the issue, whose page has since gained anchor links, keys() now raises JSONDecodeError.
• Directory entries kept a trailing / (count/), contrary to the Store.list_dir contract.

Fix

list_dir now:

• skips http(s):// entries that contain # or ?, since they name the index page rather than an object (other filesystems are unaffected, so a key containing # on S3 or local disk is still listed);
• strips the trailing /;
• drops names that are empty or still contain /;
• yields each name once.

The fsspec behaviour that turns href="/" into a self-link is arguably an fsspec bug too, but zarr shouldn't depend on it being fixed.

Test

test_list_dir_http_yields_only_children serves an index page from a local http.server with links /, ../, #, #usage, ?sort=name, a/, a, b, /group/c/, and checks that list_dir("") returns exactly a, b, c. It fails on main with ['', '#', '#usage', 'a', 'a/', ...].

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.36%. Comparing base (eb2ce59) to head (1074ff9).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4402   +/-   ##
=======================================
  Coverage   94.35%   94.36%           
=======================================
  Files          93       93           
  Lines       13131    13142   +11     
=======================================
+ Hits        12390    12401   +11     
  Misses        741      741           

Files with missing lines	Coverage Δ
src/zarr/storage/_fsspec.py	91.71% <100.00%> (+0.53%)	⬆️

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