TST: stop the isolation test from storing one wrapper twice

stochastic_calisto already holds the stochastic_nose_cone fixture, so adding
it again put the test into the state #1172 describes: one wrapper in two
entries, its position overwritten, and two reseeds landing on the same object.
The assertion looked at a different collection and passed anyway. It adds the
deterministic nose now, so add_nose builds a wrapper of its own.

Nothing pinned the body keeping the seed as given either. Reproducibility and
seed uniqueness both hold with the body on a spawned child, so neither would
have noticed it going back there.

Signed-off-by: thc1006 <84045975+thc1006@users.noreply.github.com>

Author: thc1006

rocketpy/stochastic/stochastic_rocket.py

@@ -176,7 +176,8 @@ def __init__(
             coordinate_system_orientation=None,
         )
 
-    # Nested stochastic objects, in spawn order
+    # Nested stochastic objects, in spawn order. Append rather than reorder:
+    # the position of a name here addresses its collection's stream.
     _POSITIONED_COLLECTIONS = ("aerodynamic_surfaces", "motors", "rail_buttons")
     _PLAIN_COLLECTIONS = ("parachutes", "air_brakes")
 

tests/unit/stochastic/test_stochastic_rocket_seeding.py

@@ -164,7 +164,9 @@ def parachute_draw():
         return _drawn(stochastic_calisto.parachutes[0])
 
     before = parachute_draw()
-    stochastic_calisto.add_nose(stochastic_nose_cone, position=1.1)
+    # The deterministic nose, so add_nose builds a wrapper of its own. Adding
+    # the fixture again would store one wrapper twice, which is #1172.
+    stochastic_calisto.add_nose(stochastic_nose_cone.obj, position=1.1)
 
     assert parachute_draw() == before
 
@@ -235,3 +237,15 @@ def drawn(seed):
     assert first[0] != first[1], "two air brakes drew the same value"
     assert drawn(808) == first
     assert drawn(809) != first
+
+
+def test_the_rocket_body_keeps_the_seed_as_given(monkeypatch, stochastic_calisto):
+    """Fixing the nested components did not need the body's stream to move.
+
+    Reproducibility and seed uniqueness both hold with the body on a spawned
+    child, so neither of them would notice it going back there and taking every
+    fixed-seed mass and radius baseline with it.
+    """
+    seeds = _seeds_handed_out(monkeypatch, stochastic_calisto, 42)
+
+    assert seeds[0] == 42