Preserve structured output stop reasons

[sylvesterkaczmarek]

Summary

Preserve structured-output responses when generation terminates with stop_reason="refusal" or "max_tokens" instead of replacing the response with a schema-validation exception.

messages.parse() and the beta equivalent currently attempt to validate every text block against output_format unconditionally. Refusal text and max-token-truncated JSON are not guaranteed to satisfy the requested output schema, so those terminal responses can fail inside Pydantic/JSON parsing before the caller can inspect the model's actual stop_reason.

For example, a refusal containing ordinary refusal text is currently fed to TypeAdapter.validate_json(), and a response truncated at {"value": is treated as malformed structured output. In both cases the SDK loses the more important information: why generation stopped.

Fix

Skip structured-output validation only for:

• refusal
• max_tokens

The text remains available unchanged, parsed_output is None, and the original stop_reason is preserved.

Normal completed responses continue through the existing validation path, so malformed structured output on an ordinary completed turn still raises as before.

The behavior is applied symmetrically to GA and beta parsed messages.

Regression coverage

Adds tests verifying:

• refusal text returns with parsed_output=None for GA and beta;
• truncated JSON from max_tokens returns with parsed_output=None;
• the original stop reason is preserved;
• invalid structured output on a normally completed response still raises a validation error.