Fix array shuffle index overflow for large groups - #12590

AI-assisted contribution, reviewed by @AyoobMH, who has confirmed understanding of the changes.

Array shuffle can wrap its sorting indices when a group is larger than both the input chunks and the target chunk-size limit. For example, 20-element input chunks with 257-element groups can silently return incorrect values; larger groups can instead raise the misleading unknown-chunk-size error. Include the actual output-group sizes when choosing the index dtype.

Addresses pydata/xarray#11212, reported by @joshua-gould, and extends the dtype bound introduced in #11615. The regression tests cover the uint8 boundary, uint16 overflow, both array axes, and repeated indices.

• Tests added / passed: shuffle and slicing suites — 146 passed, 2 skipped, 3 xfailed. Before the fix, seven new cases failed and the two boundary controls passed.
• Xarray reproduction and twelve diagnostic boundary/integration checks pass with the local Dask fix.
• Passes pixi run lint (not run; pinned Ruff and Black checks and git diff --check passed for the changed files).