Add regression tests for JsonSchema (#17052)

Tests added for existing JsonSchema in
`codex-rs/tools/src/json_schema_tests.rs`:

- `parse_tool_input_schema_coerces_boolean_schemas`
- `parse_tool_input_schema_infers_object_shape_and_defaults_properties`
- `parse_tool_input_schema_normalizes_integer_and_missing_array_items`
- `parse_tool_input_schema_sanitizes_additional_properties_schema`
-
`parse_tool_input_schema_infers_object_shape_from_boolean_additional_properties_only`
- `parse_tool_input_schema_infers_number_from_numeric_keywords`
- `parse_tool_input_schema_infers_number_from_multiple_of`
-
`parse_tool_input_schema_infers_string_from_enum_const_and_format_keywords`
- `parse_tool_input_schema_defaults_empty_schema_to_string`
- `parse_tool_input_schema_infers_array_from_prefix_items`
-
`parse_tool_input_schema_preserves_boolean_additional_properties_on_inferred_object`
-
`parse_tool_input_schema_infers_object_shape_from_schema_additional_properties_only`

Tests that we expect to fail on the baseline normalizer, but pass with
the new JsonSchema:

- `parse_tool_input_schema_preserves_nested_nullable_type_union`
- `parse_tool_input_schema_preserves_nested_any_of_property`