fix(ollama): declare ResponseError as fallback-eligible so the structured-output ladder can switch strategies - #3615

AgentScope Version

2.0.12dev (main @ 4acacb9)

Description

OllamaChatModel is the only mainline provider that does not override _get_structured_output_fallback_exceptions (base returns ()). In the structured-output strategy ladder (model/_base.py:565-594), an exception that is neither retryable nor fallback-eligible raises immediately — so for Ollama, any server-side rejection (ollama.ResponseError, which the SDK uses for every non-transport failure including 4xx) kills the whole ladder on the first attempt, and the auto / no_think / none strategies are never tried.

Changes — declare the fallback exception, verbatim provider style:

@classmethod
def _get_structured_output_fallback_exceptions(cls) -> tuple[Type[Exception], ...]:
    from ollama import ResponseError

    # ResponseError wraps every server-side rejection (including 4xx
    # like "model not found" or a tools/structured-output refusal) into
    # a single type, so the structured-output ladder must fall through
    # on it instead of aborting. Transport-level failures stay
    # retryable via the httpx set above.
    return (ResponseError,)

The retryable set (three httpx transport errors) stays untouched — transport failures keep retrying, protocol-level rejections now switch strategies. Same series as #3583/#3588 (Ollama thinking toggle) and #3612/#3613 (Gemini 4xx fall-through).

How to test

Regression cases added to the existing tests/model_ollama_test.py:

• test_fallback_exceptions_declare_response_error — fails on main (() != (ResponseError,));
• test_response_error_walks_the_ladder — records every _call_api attempt while the ladder degrades: on main a mocked ResponseError dies after 1 attempt; with this fix all four strategies are tried (4 attempts).

python -m pytest tests/model_ollama_test.py
# 15 passed.

Fixes #3614

Checklist

• An issue has been created for this PR
• I have read the CONTRIBUTING.md
• Docstrings are in Google style
• Related documentation has been updated (e.g. links, examples, etc.) in documentation repository
• Code is ready for review