Raise on 'majority' in zonal_stats dask path instead of silent drop (#2528)

[brendancol]

Summary

• zonal_stats() on dask-backed input silently dropped 'majority' from the requested stats list because the default included it but the dask block-by-block path could not compute it.
• Replaced the mutable list default with stats_funcs=None and resolved the default per backend; numpy/cupy keep the full eight-stat default, dask resolves to the seven-stat subset.
• Explicit 'majority' on a dask input now raises ValueError instead of being silently filtered.

Closes #2528.

Test Plan

• pytest xrspatial/tests/test_zonal.py -- 129 pass (125 pre-existing + 4 new regression tests).
• pytest xrspatial/tests/test_dasymetric.py -- 61 pass (dasymetric uses zonal.stats internally).
• Cross-backend smoke test confirms numpy/cupy keep 'majority', dask backends omit it, explicit majority on dask raises with a clear message.

[brendancol]

Review pass (worktree, fresh test run):

• (a) ValueError vs deprecation: ValueError is appropriate. There is no documented contract that majority worked on dask; the prior behaviour silently dropped it. Callers who explicitly passed 'majority' on dask were already broken in a worse way (silent data loss). No deprecation needed.
• (b) Empty list stats_funcs=[]: behaves consistently across backends; returns a frame with only the 'zone' column. The new validation does not flag it (empty unsupported list). Not adding a dedicated test, the existing subset tests cover the surrounding code path.
• (c) Docstring stats table: the per-backend default lists are now documented and the dask majority rejection is called out.
• (d) Other callers: dasymetric.py references zonal.stats only in docstrings; accessor.zonal_stats passes kwargs through; the Dataset branch recurses with the same stats_funcs (None resolves per-backend on each per-variable call). No other module passes stats_funcs explicitly.

Applied one suggestion: the dict-on-dask error message hard-coded the supported list. Sourced it from _DEFAULT_STATS_DASK to prevent drift (1 file, 2 lines).

Tests: 129/129 test_zonal.py + 150/150 dasymetric/dataset_support/validation pass.