fix: validate non-BaseModel schemas in validate_schema - #7285

Summary

Fixes #7280.

validate_schema() (src/google/adk/utils/_schema_utils.py) only ran real
validation for BaseModel and list[BaseModel] schemas. Every other
supported SchemaType — generic aliases like list[str] / dict[str, int],
raw JSON Schema dicts, and types.Schema — fell through to a bare
json.loads with no validation, so malformed values were silently accepted:

validate_schema(list[str], "[1, 2, 3]")        # -> [1, 2, 3]  (should reject)
validate_schema(dict[str, int], '{"a": "x"}')  # -> {'a': 'x'} (should reject)
validate_schema(
    {"type": "object", "properties": {"a": {"type": "string"}}, "required": ["a"]},
    '{"a": 1}',
)  # -> {'a': 1} (should reject)

This matters because output_schema accepts all of these types, and an
unvalidated result gets written straight into session state
(output_key) or returned to a parent agent via AgentTool.

Fix

• Generic alias schemas (list[str], dict[str, int], etc.) now go through
  TypeAdapter(schema).validate_json(...), the same mechanism already used
  for the list[BaseModel] branch.
• Raw JSON Schema dicts and types.Schema instances are now validated with
  jsonschema.validate (jsonschema is already a direct dependency, and
  already used elsewhere in the codebase, e.g.
  cli/built_in_agents/tools/write_config_files.py). types.Schema uses
  uppercase type names ("STRING", "OBJECT"), so it's dumped to a JSON
  Schema dict and passed through the existing lowercase_schema_types
  helper before validation.
• types.Schema.model_dump() is called with by_alias=True. types.Schema's
  Python fields are snake_case (any_of, additional_properties,
  max_items, ...) with JSON Schema keyword aliases (anyOf,
  additionalProperties, maxItems, ...). Without by_alias=True,
  model_dump() emitted the snake_case names, which jsonschema.validate
  silently ignores as unknown properties — so any_of, additionalProperties,
  and the min/max constraints were never actually enforced, even though the
  rest of the schema validated correctly. This was caught in review; added
  regression tests for any_of and additional_properties below.

No changes to pyproject.toml / lockfiles — jsonschema was already a
declared dependency.

Test plan

Added 8 tests to tests/unittests/utils/test_schema_utils.py::TestValidateSchema
covering the reject case for each affected schema kind (list[str],
dict[str, int], raw JSON Schema dict, types.Schema), the accept case
for the two new schema kinds that weren't exercised before, and two
aliased-field regression cases (any_of, additional_properties) on
types.Schema.

Confirmed the new reject-case tests fail on the pre-fix code
(git stash the source file, keep the tests, run again):

$ git stash push -- src/google/adk/utils/_schema_utils.py
$ pytest tests/unittests/utils/test_schema_utils.py -q
...
FAILED ...::test_list_of_str_schema_rejects_wrong_element_type - Failed: DID NOT RAISE ValidationError
FAILED ...::test_dict_schema_rejects_wrong_value_type - Failed: DID NOT RAISE ValidationError
FAILED ...::test_raw_json_schema_dict_rejects_invalid_value - Failed: DID NOT RAISE ValidationError
FAILED ...::test_types_schema_rejects_invalid_value - Failed: DID NOT RAISE ValidationError
FAILED ...::test_types_schema_any_of_rejects_invalid_value - Failed: DID NOT RAISE ValidationError
FAILED ...::test_types_schema_additional_properties_rejects_extra_field - Failed: DID NOT RAISE ValidationError
6 failed, 65 passed
$ git stash pop

With the fix restored:

$ pytest tests/unittests/utils/test_schema_utils.py -q
.......................................................................    [100%]
71 passed in 0.80s

Also ran the broader suite for callers of validate_schema plus the rest of
tests/unittests/utils/, all green:

$ pytest tests/unittests/agents/test_llm_agent_output_save.py tests/unittests/tools/test_agent_tool.py tests/unittests/utils/ -q
552 passed, 209 warnings in 7.31s

pre-commit run passes on the changed files (ruff, isort, pyink, addlicense,
codespell). mypy reports the same 6 pre-existing errors in this file with
and without the change (unrelated to this fix, present on unmodified main).

AI disclosure

This change was written with the assistance of Claude Code (Anthropic).

🤖 Generated with Claude Code