fix(labels): defer float-dtype rejection to spatialdata's model validation

[timtreis]

Problem

main is red on 4 label-related tests, all caused by spatialdata 0.8:

• test_render_labels_rejects_float_dtype[float16/32/64]
• test_utils.py::TestMeasureObs::test_float_dtype_labels_supported

They fail at test setup, with:

ValueError: Labels must have an integer dtype, found float32. Cast the data, e.g. `.astype(np.uint16)`.

Root cause

#606 (filed against spatialdata 0.5, where Labels2DModel.parse still accepted float32) added a float-dtype guard inside _render_labels. spatialdata 0.8 moved that validation to the model boundary — parse(), SpatialData(...), and sd[k] = ... all now reject non-integer label rasters with a clear, actionable message (verified: the only way to obtain a float-labels element is a raw .data mutation, which is unsupported).

Consequences:

• The render-time guard is now redundant dead code — a validly built element can never reach the renderer with a float dtype, and upstream raises earlier and better (exactly the boundary render_labels crashes with cryptic skimage TypeError when label array has float dtype #606 asked for).
• Both tests fail at setup because they can no longer construct a float-labels element.

Changes

• Remove the redundant float-dtype guard in _render_labels; defer to spatialdata's boundary validation (interop over duplication).
• Drop test_render_labels_rejects_float_dtype — it asserts an unreachable render-time scenario.
• Rewrite the measure_obs float test to exercise _stream_label_centroid_stats directly (where the integer-valued-float → int cast actually lives), asserting a float raster yields exactly what its integer counterpart yields. The cast itself is kept as cheap correct-by-construction robustness.

Not in scope

The 5th main failure — Shapes_…random_shuffling (visual) — is a separate, deterministic upstream draw-order drift (colors still map correctly; only overlap z-order changed). Tracked for its own baseline-refresh PR, not folded in here.

Testing

• Full non-visual test_utils.py (140) and test_render_labels.py pass locally.
• The two obsolete test IDs are removed/renamed; the new helper-level test covers the retained cast.