fix: honor mask/alpha bands and mask RGB nodata only when all bands match

[TommySquared]

Problem

_process_rgb treats a pixel as nodata when any of its three bands equals the nodata value. In a file tagged nodata=0, that punches holes in valid pixels that happen to have one zero channel: pure red (255, 0, 0), deep shadows, saturated composites. A fill value fills every band, so the rule should require all three.

Neither _process_rgb nor _process_singleband reads the file's mask band or alpha band. Only the nodata tag and NaN are used, so GeoTIFFs that mark empty pixels with a GDAL internal mask or an alpha band (common in GIS exports) draw those pixels opaque.

Changes

• RGB nodata rule: transparent only when all three bands equal the nodata value (tag or nodata override). NaN in any band still masks, since that pixel has no color to draw.
• Mask and alpha bands: both RGB and single-band now honor a per-dataset mask or alpha band via read_masks. Nodata tags are handled exactly as before, so a nodata override still replaces the tag. Files without a mask render as they did.
• NaN cast: the RGB stretch zeroes masked pixels before the uint8 cast. NaN was reaching astype(np.uint8) (undefined behavior, hidden by the alpha channel, but it raised RuntimeWarning: invalid value encountered in cast).

Behavior change to note: a file that has both a nodata tag and a mask band now hides the union of the two.

Testing

• New server/tests/test_geotiff_masking.py (7 tests): pure red and shadow pixels stay opaque under nodata=0, all-zero fill is hidden, the override uses the same rule, NaN in one band masks, internal mask and alpha band are honored, and a file with no tag or mask is fully opaque. 5 of the 7 fail on current main.
• Tested by hand against a running stack built from this branch.
• The rest of the suite shows the same results as on main locally: most tests error with StreamableHTTPSessionManager .run() can only be called once per instance from the MCP test fixtures, which is unrelated to this change. No existing test covers geotiff_service.py.

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