Establish the copick-shared-ui migration test and fixture gate

[uermel]

Parent epic: #53

Objective

Establish a real storage-sensitive test and packaging gate before changing copick-shared-ui's runtime dependencies.

Work

• Add pytest and offscreen Qt configuration and a workflow that fails if zero tests collect.
• Preserve the current Zarr 2 production runtime for this baseline phase.
• Add independently constructed OME-Zarr 0.4 / Zarr v2 and OME-Zarr 0.5 / Zarr v3 twins with numeric and nonnumeric metadata paths.
• Reproduce the current bug with metadata selecting s0 while a distracting array named 0 exists.
• Cover unsharded, one-shard, multi-shard, alternate chunk, alternate key, and supported alternate codec layouts.
• Add invalid fixtures for empty and malformed stores, missing paths, root arrays, and non-3D arrays.
• Verify metadata selection, read-only behavior, source immutability, and error propagation through runtime tests; do not add AST parsing or structural source-guard tests.
• Align the formatter used by pre-commit, the lock, and CI.
• Run the baseline on Python 3.11 through 3.14, with the dependency-floor and locked jobs required by the migration plan.

Acceptance criteria

• The current metadata-selection defect is one explicit expected failure.
• All other baseline tests pass without production storage changes.
• Fixtures do not use the helper they are intended to test.
• CI fails if pytest collects no tests.
• Lock, lint, format, compile/import, build, and artifact checks pass.

Release context

This work starts immediately. It does not wait for another core release: copick 2.0.0-alpha.1 is already published.

[uermel]

Implementation update from the migration PR stack:

• test: establish shared UI migration gate #54 provides the real pytest/offscreen-Qt gate and cache isolation.
• feat!: adopt metadata-driven Zarr 3 thumbnails #55 contains the metadata-driven runtime change plus the v2/v3, invalid-store, and layout fixtures already implemented in the stack.
• At maintainer request, the AST-parsing and structural source-guard tests have been removed. The issue and migration plan now call for observable runtime behavior tests instead.
• test: establish shared UI migration gate #54 does not close this issue early; both PRs use Part of #49.

Keeping this issue open for final reconciliation of its broader matrix, notably the supported alternate-codec fixture and the remaining cache-bypass, info/gallery-model, CLI-schema, and URI smoke coverage called out during review.