OpenAPI tool: generate_return_doc crashes on non-numeric response keys (default, NXX)

[garyzava]

Describe the bug

PydocHelper.generate_return_doc builds its sorted response list with
sorted(responses.items(), key=lambda item: int(item[0])). OpenAPI 3.x allows
non-numeric keys in the Responses Object — default and the range patterns
1XX/2XX/3XX/4XX/5XX. When any of those keys is present, int(item[0])
raises ValueError, so generating a tool from an otherwise valid OpenAPI spec
crashes.

To Reproduce

from fastapi.openapi.models import Response, Schema, MediaType
from google.adk.tools.openapi_tool.common.common import PydocHelper

responses = {
    "200": Response(description="OK", content={"application/json": MediaType(schema=Schema(type="string"))}),
    "default": Response(description="Error response"),
}
PydocHelper.generate_return_doc(responses)
# ValueError: invalid literal for int() with base 10: 'default'

Expected behavior

Non-numeric response keys should be ignored for return-type hinting (the code
already keeps only 2xx responses with content on the very next line), so the
function should return the doc for the 200 response instead of crashing.

Desktop

• google-adk 2.1.0 and later; still present on main as of 2026-06-10

Additional context

operation_parser already filters startswith('2') before consuming the keys;
applying the same guard here keeps the two paths consistent.