Remove dead code: simplify_nullable_unions feature

GoogleJsonSchemaTransformer was the only user of simplify_nullable_unions=True.
After removing it (because Google now supports type: 'null' natively), this feature
became completely unused, causing coverage to drop to 99.97%.

Removed:
- simplify_nullable_unions parameter from JsonSchemaTransformer.__init__
- self.simplify_nullable_unions assignment
- Conditional call to _simplify_nullable_union() in _handle_union()
- Entire _simplify_nullable_union() static method (18 lines)

Verified no other references exist in the codebase.
This restores 100% test coverage.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: conradlee

pydantic_ai_slim/pydantic_ai/_json_schema.py

@@ -25,15 +25,13 @@ def __init__(
         *,
         strict: bool | None = None,
         prefer_inlined_defs: bool = False,
-        simplify_nullable_unions: bool = False,
     ):
         self.schema = schema
 
         self.strict = strict
         self.is_strict_compatible = True  # Can be set to False by subclasses to set `strict` on `ToolDefinition` when set not set by user explicitly
 
         self.prefer_inlined_defs = prefer_inlined_defs
-        self.simplify_nullable_unions = simplify_nullable_unions
 
         self.defs: dict[str, JsonSchema] = self.schema.get('$defs', {})
         self.refs_stack: list[str] = []
@@ -146,10 +144,6 @@ def _handle_union(self, schema: JsonSchema, union_kind: Literal['anyOf', 'oneOf'
 
         handled = [self._handle(member) for member in members]
 
-        # convert nullable unions to nullable types
-        if self.simplify_nullable_unions:
-            handled = self._simplify_nullable_union(handled)
-
         if len(handled) == 1:
             # In this case, no need to retain the union
             return handled[0] | schema
@@ -159,26 +153,6 @@ def _handle_union(self, schema: JsonSchema, union_kind: Literal['anyOf', 'oneOf'
         schema[union_kind] = handled
         return schema
 
-    @staticmethod
-    def _simplify_nullable_union(cases: list[JsonSchema]) -> list[JsonSchema]:
-        # TODO: Should we move this to relevant subclasses? Or is it worth keeping here to make reuse easier?
-        if len(cases) == 2 and {'type': 'null'} in cases:
-            # Find the non-null schema
-            non_null_schema = next(
-                (item for item in cases if item != {'type': 'null'}),
-                None,
-            )
-            if non_null_schema:
-                # Create a new schema based on the non-null part, mark as nullable
-                new_schema = deepcopy(non_null_schema)
-                new_schema['nullable'] = True
-                return [new_schema]
-            else:  # pragma: no cover
-                # they are both null, so just return one of them
-                return [cases[0]]
-
-        return cases
-
 
 class InlineDefsJsonSchemaTransformer(JsonSchemaTransformer):
     """Transforms the JSON Schema to inline $defs."""