Honor RGBA alpha in render_labels, aligning with points/shapes behavior

Change fill_alpha default from 0.4 to None so that _type_check_params
can extract the alpha from an RGBA color tuple, matching how points and
shapes already handle this.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: timtreis

src/spatialdata_plot/pl/basic.py

@@ -640,7 +640,7 @@ def render_labels(
         norm: Normalize | None = None,
         na_color: ColorLike | None = "default",
         outline_alpha: float | int = 0.0,
-        fill_alpha: float | int = 0.4,
+        fill_alpha: float | int | None = None,
         scale: str | None = None,
         colorbar: bool | str | None = "auto",
         colorbar_params: dict[str, object] | None = None,
@@ -689,8 +689,9 @@ def render_labels(
             won't be shown.
         outline_alpha : float | int, default 0.0
             Alpha value for the outline of the labels. Invisible by default.
-        fill_alpha : float | int, default 0.4
-            Alpha value for the fill of the labels.
+        fill_alpha : float | int | None, optional
+            Alpha value for the fill of the labels. By default, it is set to 0.4 or, if a color is given that implies
+            an alpha, that value is used for `fill_alpha`.
         scale :  str | None
             Influences the resolution of the rendering. Possibilities for setting this parameter:
                 1) None (default). The image is rasterized to fit the canvas size. For multiscale images, the best scale

src/spatialdata_plot/pl/utils.py

@@ -2270,6 +2270,9 @@ def _type_check_params(param_dict: dict[str, Any], element_type: str) -> dict[st
     elif element_type == "shapes":
         # set default fill_alpha for shapes if not given by user explicitly or implicitly (as part of color)
         param_dict["fill_alpha"] = 1.0
+    elif element_type == "labels":
+        # set default fill_alpha for labels if not given by user explicitly or implicitly (as part of color)
+        param_dict["fill_alpha"] = 0.4
 
     cmap = param_dict.get("cmap")
     palette = param_dict.get("palette")
@@ -2427,7 +2430,7 @@ def _validate_label_render_params(
     element: str | None,
     cmap: list[Colormap | str] | Colormap | str | None,
     color: ColorLike | None,
-    fill_alpha: float | int,
+    fill_alpha: float | int | None,
     contour_px: int | None,
     groups: list[str] | str | None,
     palette: list[str] | str | None,