fix: validate consolidated metadata listing keys and hierarchy

[barlowa124]

Summary

Fixes the second item in #4453: ConsolidatedMetadata.from_dict (and
GroupMetadata.from_dict, which delegates to it) accepted consolidated
listings the spec rules out, and two of them broke reads.

• Listing keys are node paths. Keys with an empty segment ("", a//b) or
  a reserved segment (., .., zarr.json, __*) are now rejected with
  MetadataValidationError before any member is parsed.
• Every non-root key must have all ancestors present as group entries.
  {"a/b": GROUP} used to raise KeyError: 'a', which surfaced as
  GroupNotFoundError on open_group. The root group became unopenable
  because of one entry. It now fails with a clear MetadataValidationError.
  A listing below an array (which members() hit as KeyError) is
  rejected the same way.

The remaining #4453 items (float fill equality, fill-value edges,
document-level member validation) are untouched.

For reviewers

The stricter empty-segment rule is deliberate: consolidated keys are node
paths made of node names, and node names may not be empty. This differs
from #4484's name check, which skips empty segments so "/foo" and
a//b keep their existing create-path normalization. If #4484 lands
first this can reuse its helper for the reserved-name portion.

Scope check: zarr_format: 2 entries inside a v3 listing are still
accepted; the issue lists it among the accepted-invalid cases but the
right handling for v2/v3 mixing is less clear-cut than the path rules, so
it is left out of this PR.

Author attestation

• I am a human, these are my changes, and I have reviewed and understood every change and can explain why each is correct.

TODO

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/user-guide/*.md
• Changes documented as a new file in changes/
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.70%. Comparing base (069fd20) to head (7b01078).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4485   +/-   ##
=======================================
  Coverage   94.69%   94.70%           
=======================================
  Files          94       94           
  Lines       13606    13626   +20     
=======================================
+ Hits        12884    12904   +20     
  Misses        722      722           

Files with missing lines	Coverage Δ
src/zarr/core/group.py	95.76% <100.00%> (+0.08%)	⬆️

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