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

[d-v-b]

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Bundles the minor findings from the PR zarr-developers#3885 audit — six independent items, one commit:

1. Duplicated codec warnings: 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 chain now use a quiet codecs_from_list_unchecked variant (named function per the two-semantics-two-names convention, no global warning-filter mutation). Regression test asserts exactly one warning per open under both pipelines; direct from_codecs construction still warns once.
2. concurrent_iter laziness: it returned a generator of yet-to-be-created tasks while its docstring promised eager scheduling — a serialize-on-await trap for future callers. Now materializes the task list, with a flag-based eagerness test.
3. max_workers env parsing: a garbage ZARR_CODEC_PIPELINE__MAX_WORKERS raised ValueError mid-read; now warns and falls back to the default (tolerant handling for config/env input).
4. Abandoned in-flight tasks: the as-completed pipeline helpers left already-spawned fetch/write tasks running when one failed — stray background writes plus "Task exception was never retrieved" noise. Failures now cancel and drain outstanding tasks (single drain over the concatenated task sets, so outer cancellation can't skip half).
5. Benchmark hygiene: seeded data generator (reproducible runs), fixed a copy-pasted docstring.
6. Removed dead commented-out set_range_sync test blocks left behind by the pre-merge removal of that API.

One review note worth knowing: BatchedCodecPipeline.evolve_from_array_spec now propagates self.batch_size instead of re-resolving config — verified safe (all production callers construct via from_codecs, which resolves config at that point; the sharding inner-pipeline cache already keys on the config value).

Verification: warning test passes for both pipelines (fused failed pre-fix, TDD); 97 + 2042 targeted tests green; full repo suite 6589 passed pre-fix-round with the same items; mypy/ruff clean. Two-round task review (initial review caught the fused double-warn and a thread-safety hazard in the first approach; re-review verified both closed with no new issues).

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[d-v-b]

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Merge-order note: this branch has small textual conflicts with #254 in src/zarr/core/chunk_utils.py and tests/test_fused_pipeline.py (adjacent edits). Whichever merges second needs a trivial rebase — ask and I'll do it.

[d-v-b]

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Update: after #254 was narrowed (its async-only-inner-codec fix deferred to upstream zarr-developers#4179), the chunk_utils.py conflict is gone; only the trivial tests/test_fused_pipeline.py adjacent-append conflict with #254 remains.