fix: minor correctness and hygiene fixes from the sync-pipeline audit

[d-v-b]

A variety of fixes exposed after claude did an audit of #3885

🤖 AI text below 🤖

• Codec construction warnings (e.g. sharding's "disables partial reads") fired twice per array open, and on every decode/encode through the fused pipeline's async fallback. Re-constructions of an already-validated codec chain now go through codecs_from_list_unchecked, which validates structure without repeating first-construction advisory warnings; each warning fires exactly once per open under both pipelines.
• concurrent_iter returned a lazy generator while its docstring promised eagerly scheduled tasks; it now materializes the task list so awaiting one at a time cannot serialize the batch.
• A garbage codec_pipeline.max_workers value (e.g. from the environment) raised ValueError mid-read; it now warns and falls back to the default, consistent with tolerant handling of config input.
• The as-completed pipeline helpers abandoned in-flight tasks when one failed, leaving stray background writes and "Task exception was never retrieved" warnings; failures now cancel and drain outstanding tasks.
• Benchmarks: seed the data generator for reproducibility; fix a copy-pasted docstring.
• Remove dead commented-out test blocks referencing the removed set_range API.

Assisted-by: ClaudeCode:claude-sonnet-5

Based on this claude-authored PR: d-v-b#259

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

❌ Patch coverage is 97.77778% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 93.92%. Comparing base (a88f889) to head (cdb0e00).

Files with missing lines	Patch %	Lines
src/zarr/core/codec_pipeline.py	97.56%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4205      +/-   ##
==========================================
+ Coverage   93.91%   93.92%   +0.01%     
==========================================
  Files          91       91              
  Lines       12723    12744      +21     
==========================================
+ Hits        11949    11970      +21     
  Misses        774      774              

Files with missing lines	Coverage Δ
src/zarr/core/array.py	97.85% <ø> (ø)
src/zarr/core/chunk_utils.py	98.27% <100.00%> (ø)
src/zarr/core/common.py	88.75% <100.00%> (ø)
src/zarr/core/codec_pipeline.py	96.37% <97.56%> (+0.16%)	⬆️

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[d-v-b]

these are internal changes that improve the usability of the new fusedcodecpipeline, so I will self-merge