Closes #25114.

Previously polygons generated by tricontourf have finished with a kind code of 2 (MOVETO) rather than 79 (CLOSEPOLY). This wasn't a problem for rendering but was for people doing something with the generated paths such as using to_polygons().

The fix is to ensure that polygons generated by tricontour and tricontourf have their last vertex identical to their first vertex, and the correct CLOSEPOLY code. This is the same as that produced by contour and contourf.

Simple reproducer:

import matplotlib.tri as mtri
import matplotlib.pyplot as plt

x = [0, 1, 1, 0, 0.5]
y = [0, 0, 1, 1, 0.5]
z = [0, 0, 0, 0, 1.0]
triangles = [[0, 1, 4], [1, 2, 4], [2, 3, 4], [3, 0, 4]]
triang = mtri.Triangulation(x, y, triangles)

_, ax = plt.subplots()
cs = ax.tricontourf(triang, z, levels=[0.5, 2])
print(cs.get_paths()[0])

Before the fix this gives

Path(array([[0.75, 0.25],
            [0.75, 0.75],
            [0.25, 0.75],
            [0.25, 0.25]]), array([1, 2, 2, 2], dtype=uint8))

and after the fix

Path(array([[0.75, 0.25],
            [0.75, 0.75],
            [0.25, 0.75],
            [0.25, 0.25],
            [0.75, 0.25]]), array([ 1,  2,  2,  2, 79], dtype=uint8))

(Edited to avoid using deprecated API)