Remove additional dead code: single-member union collapse path

After removing simplify_nullable_unions, the code path that handled unions
collapsing to a single member (line 147-149) became unreachable.

This path was only hit when simplify_nullable_unions converted:
  anyOf: [{type: 'string'}, {type: 'null'}] → {type: 'string', nullable: true}

Without that feature, multi-member unions can't collapse to 1 member naturally.

Removed 4 more lines of unreachable code.
This should restore 100% 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

@@ -144,10 +144,6 @@ def _handle_union(self, schema: JsonSchema, union_kind: Literal['anyOf', 'oneOf'
 
         handled = [self._handle(member) for member in members]
 
-        if len(handled) == 1:
-            # In this case, no need to retain the union
-            return handled[0] | schema
-
         # If we have keys besides the union kind (such as title or discriminator), keep them without modifications
         schema = schema.copy()
         schema[union_kind] = handled