Failed Interactions API calls always report UNKNOWN_ERROR / "Unknown error" #7882

🔴 Required Information

Describe the Bug:
With Gemini(use_interactions_api=True), every failed interaction comes back as error_code="UNKNOWN_ERROR" and error_message="Unknown error". The real error, such as RESOURCE_EXHAUSTED: Quota exceeded…, never reaches the caller, so retries, on_model_error_callback and logs can't tell a quota error from a bad request.

convert_interaction_to_llm_response reads interaction.error, but no google-genai Interaction declares an error field. The API puts failures in errors: list[Error(code, message)]. On a completed SSE event, the partial InteractionSseEventInteraction keeps the same undeclared errors as raw dicts. So getattr(interaction, 'error', None) is always None, and the defaults win. This affects non-streaming calls, deferred-tier polling and streaming.

build_interactions_response_log reads the same missing field, so the debug log of a failed interaction always shows Error: (none).

The existing test_failed_response doesn't catch this, because it sets interaction.error = MagicMock(...) by hand.

Steps to Reproduce:

1. Use Gemini(model="gemini-3-flash-preview", use_interactions_api=True).
2. Have the API return a failed interaction, e.g. {"id": "int-9", "status": "failed", "errors": [{"code": "RESOURCE_EXHAUSTED", "message": "Quota exceeded for model gemini-3-flash-preview"}]}.

Expected Behavior:
LlmResponse.error_code == "RESOURCE_EXHAUSTED" and error_message == "Quota exceeded for model gemini-3-flash-preview".

Observed Behavior:
With a real google-genai client (2.34.0) and an httpx.MockTransport returning the payload above:

non_streaming  -> error_code='UNKNOWN_ERROR' error_message='Unknown error'
deferred       -> error_code='UNKNOWN_ERROR' error_message='Unknown error'
streaming      -> error_code='UNKNOWN_ERROR' error_message='Unknown error'

Environment Details:

• ADK Library Version (pip show google-adk): 2.13.0 (also on main at e9e8f61)
• Desktop OS: macOS
• Python Version (python -V): 3.11.16

Model Information:

• Are you using LiteLLM: No
• Which model is being used: Gemini (gemini-3-flash-preview) through the Interactions API

🟡 Optional Information

Minimal Reproduction Code:

from google.adk.models import interactions_utils
from google.genai.interactions import Error, Interaction

interaction = Interaction(
    id="int-9",
    status="failed",
    errors=[Error(code="RESOURCE_EXHAUSTED", message="Quota exceeded")],
)
r = interactions_utils.convert_interaction_to_llm_response(interaction)
print(r.error_code, r.error_message)  # UNKNOWN_ERROR Unknown error

How often has this issue occurred?:

• Always (100%), for every failed interaction