test: Remove xfail markers for describe() tests now that nullable is preserved

The describe() method now correctly preserves nullable modifiers on FK
attributes, so these tests should pass.

Removed xfail from:
- test_declare.py::test_describe
- test_declare.py::test_describe_indexes
- test_declare.py::test_describe_dependencies
- test_foreign_keys.py::test_describe

Note: test_json.py::test_describe still has xfail for a separate issue
(index reconstruction).

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: dimitri-yatsenko

tests/integration/test_declare.py

@@ -40,7 +40,6 @@ def test_instance_help(schema_any):
     assert TTest2().definition in TTest2().__doc__
 
 
-@pytest.mark.xfail(reason="describe() doesn't preserve NOT NULL on FK attributes - pre-existing bug")
 def test_describe(schema_any):
     """real_definition should match original definition"""
     rel = Experiment()
@@ -51,7 +50,6 @@ def test_describe(schema_any):
     assert s1[0] == s2[0]  # Compare SQL only (declare now returns tuple)
 
 
-@pytest.mark.xfail(reason="describe() doesn't preserve NOT NULL on FK attributes - pre-existing bug")
 def test_describe_indexes(schema_any):
     """real_definition should match original definition"""
     rel = IndexRich()
@@ -62,7 +60,6 @@ def test_describe_indexes(schema_any):
     assert s1[0] == s2[0]  # Compare SQL only (declare now returns tuple)
 
 
-@pytest.mark.xfail(reason="describe() doesn't preserve NOT NULL on FK attributes - pre-existing bug")
 def test_describe_dependencies(schema_any):
     """real_definition should match original definition"""
     rel = ThingC()

tests/integration/test_foreign_keys.py

@@ -1,5 +1,3 @@
-import pytest
-
 from datajoint.declare import declare
 
 from tests.schema_advanced import (
@@ -29,7 +27,6 @@ def test_aliased_fk(schema_adv):
     assert delete_count == 16
 
 
-@pytest.mark.xfail(reason="describe() doesn't preserve NOT NULL on FK attributes - pre-existing bug")
 def test_describe(schema_adv):
     """real_definition should match original definition"""
     for rel in (LocalSynapse, GlobalSynapse):