Added guard against annotated instance_id==0 elements in render_labels

[SAY-5]

Closes #607.

When the annotation table for render_labels includes a row with instance_id=0, downstream masking would fail with IndexError: Boolean index has wrong length from inside the color-vector path: SpatialData's foreground-only get_values returned one fewer entry than the table's instance_id array, and the mask at render.py:1673 mismatched. Nothing in the traceback mentioned background label 0.

Added an explicit check after the table is subset to the element: if instance_id=0 is present, raise a ValueError naming the table, element, and the fix (remove the row). Regression test in test_render_labels.py.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.50%. Comparing base (53abe71) to head (f3eea16).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #660   +/-   ##
=======================================
  Coverage   77.50%   77.50%           
=======================================
  Files          11       11           
  Lines        3569     3574    +5     
  Branches      837      839    +2     
=======================================
+ Hits         2766     2770    +4     
  Misses        481      481           
- Partials      322      323    +1     

Files with missing lines	Coverage Δ
src/spatialdata_plot/pl/render.py	87.34% <100.00%> (+0.03%)	⬆️

... and 2 files with indirect coverage changes

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