fix(genai): preserve format/example fields and support integer enums in schema conversion

[Alon Nahmias (alonahmias)]

Problem

While debugging why a Gemini agent kept sending a bare date ("2026-08-17") instead of the RFC 3339 datetime declared in a tool's OpenAPI schema (format: "date-time", with a matching example), I traced the root cause to langchain_google_genai/_function_utils.py's schema conversion path (_dict_to_genai_schema / _get_properties_from_schema):

1. format is silently dropped for every property. _dict_to_genai_schema builds schema_dict from formatted_schema, but never copies formatted_schema["format"] into it — even though "format" is in _ALLOWED_SCHEMA_FIELDS and reaches _format_json_schema_to_gapic untouched. So a property declared with format: "date-time" never actually reaches the Gemini API call.
2. example/examples are dropped entirely. They aren't in _ALLOWED_SCHEMA_FIELDS at all, even though google.genai.types.Schema has a native example: Optional[Any] field:

   example: Optional[Any] = Field(
       default=None,
       description="Optional. Example of the object. Will only populated when the object is the root.",
   )

3. Integer/number/boolean enums are silently invalid. Schema.enum's own docstring documents the expected shape for non-string enums:

   2. We can define apartment number as: {type:INTEGER, format:"enum", enum:["101", "201", "301"]}
   

   i.e. format: "enum" must be set and enum values must be strings even when type is INTEGER/NUMBER/BOOLEAN. The current conversion just copies the raw (integer-typed) enum list and never sets format, so e.g. Literal[100, 110, 120] produces an enum Gemini doesn't recognize correctly.

Also found and removed a redundant duplicate if "enum" in schema: items["enum"] = schema["enum"] in _get_items_from_schema — harmless before this change, but it would have clobbered the new stringified array-item enum with the raw values.

Fix

• _dict_to_genai_schema: copy format and example from formatted_schema into schema_dict.
• _ALLOWED_SCHEMA_FIELDS: add "example"/"examples"; _format_json_schema_to_gapic collapses plural JSON Schema examples to the singular Schema.example.
• _get_properties_from_schema / _get_items_from_schema: propagate format generally, and for INTEGER/NUMBER/BOOLEAN types with an enum, stringify the enum values and set format: "enum", matching Gemini's documented requirement.

Testing

• Added test_tool_field_format_is_preserved, test_tool_field_example_is_preserved, test_tool_field_integer_enum to tests/unit_tests/test_function_utils.py.
• uv run pytest tests/unit_tests — 385 passed (was 382 before, +3 new tests).
• uv run mypy langchain_google_genai/ — no issues.
• uv run ruff format / ruff check — clean.

No existing tests changed behavior; this only adds previously-dropped fields and fixes previously-invalid enum output.