Don't allow negative contour values

[dstansby]

Fixes/Closes

Fixes #5829

Description

This limits the contour spin box limits to exclude negative numbers. Negative values do not have any affect on the contouring and act the same as 0, so I think it makes sense to prevent users from setting negative values to avoid confusion.

I couldn't obviously think of a test to add for this, but happy to dig further if required.

References

Type of change

• Bug-fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

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• example: all tests pass with my change
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  as there are small differences between the two Qt bindings.

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[brisvag]

Makes sense to me!

[Czaki]

It should also be fixed here:

napari/napari/layers/labels/labels.py

Lines 407 to 411 in 3868f5c

	@contour.setter
	def contour(self, contour):
	self._contour = contour
	self.events.contour()
	self.refresh()

Im not sure if silent fixing should be applied or raise an exception.

[dstansby]

It looks like in other places (e.g. color_mode, contrast_limits) errors are raised on invalid values, so I'll raise an error.

[codecov]

Codecov Report

Merging #5830 (67999b8) into main (3868f5c) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #5830      +/-   ##
==========================================
+ Coverage   89.86%   89.92%   +0.05%     
==========================================
  Files         615      615              
  Lines       52474    52478       +4     
==========================================
+ Hits        47158    47189      +31     
+ Misses       5316     5289      -27     

Impacted Files	Coverage Δ
napari/_qt/layer_controls/qt_labels_controls.py	92.07% <100.00%> (ø)
napari/layers/labels/_tests/test_labels.py	99.51% <100.00%> (+<0.01%)	⬆️
napari/layers/labels/labels.py	95.87% <100.00%> (+0.01%)	⬆️

... and 7 files with indirect coverage changes