MNT: self-document pressure_ISA bounds and strengthen discretization test

Derive the pressure_ISA discretization bounds from the standard-atmosphere
layer table (geopotential_height[0]/[-1]) instead of hardcoding -2000/80000,
and document why the grid is split around sea level. Strengthen the
discretization test with physical-sanity assertions (strictly increasing
altitude, strictly decreasing pressure, sea level sampled). No change in
numeric output.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: Gui-FernandesBR

rocketpy/environment/environment.py

@@ -2554,9 +2554,19 @@ def pressure_function(h):
                 )
             return P
 
-        # Discretize this Function to speed up the trajectory simulation
-        min_height = geopotential_height_to_geometric_height(-2000, earth_radius)
-        max_height = geopotential_height_to_geometric_height(80000, earth_radius)
+        # Discretize this Function to speed up the trajectory simulation.
+        # The ISA model is only defined between the lowest and highest
+        # geopotential layers, so convert those bounds to geometric height and
+        # sample the whole valid range (including below sea level, which the
+        # previous grid starting at 0 m did not cover).
+        min_height = geopotential_height_to_geometric_height(
+            geopotential_height[0], earth_radius
+        )
+        max_height = geopotential_height_to_geometric_height(
+            geopotential_height[-1], earth_radius
+        )
+        # Split the 100 samples between the narrow sub-sea-level band and the
+        # much wider region above it.
         altitudes_below = np.linspace(min_height, 0, 10, endpoint=False)
         altitudes_above = np.linspace(0, max_height, 90)
         altitudes = np.concatenate((altitudes_below, altitudes_above))

tests/unit/environment/test_environment.py

@@ -831,27 +831,35 @@ def test_pressure_conversion_factor_autodetect_by_model(
 
 
 def test_pressure_isa_discretization_bounds(example_plain_env):
-    """Test that pressure_ISA contains the expected range of altitudes
-    starting from the minimum geopotential height (-2000m) converted to
-    geometric height, up to the maximum (80000m) geopotential height converted to
-    geometric height.
+    """The pressure_ISA discretization must span the full range of the
+    Standard Atmosphere model: from the lowest geopotential layer (-2000 m) up
+    to the highest (80000 m), both converted to geometric height. It must also
+    be a physically sane pressure curve: altitude strictly increasing, pressure
+    strictly decreasing, and sea level (0 m) sampled exactly.
     """
     from rocketpy.tools import geopotential_height_to_geometric_height
 
     # Act
     pressure_isa_function = example_plain_env.pressure_ISA
     source_array = pressure_isa_function.source
     altitudes = source_array[:, 0]
+    pressures = source_array[:, 1]
 
     # Expected min/max geometric heights
     earth_radius = example_plain_env.earth_radius
     expected_min_height = geopotential_height_to_geometric_height(-2000, earth_radius)
     expected_max_height = geopotential_height_to_geometric_height(80000, earth_radius)
 
     # Assert
+    assert len(altitudes) == 100
     assert np.isclose(altitudes[0], expected_min_height)
     assert np.isclose(altitudes[-1], expected_max_height)
-    assert len(altitudes) == 100
+    assert expected_min_height < 0 < expected_max_height
+    # Sea level must be one of the sampled points (split boundary)
+    assert np.any(np.isclose(altitudes, 0.0))
+    # Physical sanity: altitude increasing, pressure decreasing monotonically
+    assert np.all(np.diff(altitudes) > 0)
+    assert np.all(np.diff(pressures) < 0)
 
 
 @pytest.mark.parametrize(