Use a safe weed-detection buffer default

[Renato L. de F. Cunha (renatolfc)]

Weed detection defaults buffer to -50 in raster CRS units. For the WGS84 raster attached to #139, that means negative 50 degrees and collapses the boundary before rasterization.

This PR makes the generic default zero while preserving explicit projected-unit buffers. It also rejects a buffer that creates empty or invalid geometry before rasterization. A focused WGS84 regression covers both paths.

Closes #139.

[Copilot]

Pull request overview

Adjusts the weed-detection op’s default boundary buffering to be safe for geographic (WGS84) rasters, avoiding negative-degree buffering that can collapse geometries and break rasterization. This aligns the op’s defaults with the intent that buffers are meaningful primarily in projected CRS units.

Changes:

• Change the weed-detection op default buffer from -50 to 0.
• Add a pre-rasterization validation step that rejects buffers producing empty/invalid geometries in the raster CRS.
• Add a focused WGS84 regression test covering the new default and the invalid-buffer rejection path.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
ops/weed_detection/weed_detection.yaml	Updates the default buffer parameter to 0 to avoid unsafe behavior in geographic CRS.
ops/weed_detection/weed_detection.py	Validates buffered geometry before rasterization and reuses the buffered geometry for mask generation.
ops/weed_detection/test_weed_detection.py	Adds regression coverage for WGS84 default buffer behavior and invalid-buffer error handling.

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