[Bug] Capability::OUTPUT_STRUCTURED is incorrectly listed for Gemini < 2.5 and lacks a proper exception

[Griffon-Weglot]

Hello @OskarStark, @chr-hertel,

I'm encountering an issue with the output_structure option (which maps to Capability::OUTPUT_STRUCTURED) when using Gemini models, specifically versions 2.0.

Problem Description
When trying to use the AI Agent with a gemini-2.0 model and the output_structure option to get a formatted JSON response, the call does not work as expected.

Upon investigation, I checked the official Google AI documentation regarding structured output (JSON Schema): https://ai.google.dev/gemini-api/docs/structured-output?hl=en

This documentation is very clear and states (regarding JSON Schema support via responseJsonSchema):

"It only works with Gemini 2.5."

This confirms that Gemini 2.0 models (like gemini-2.0-flash, etc.) do not support this feature.

Issues Identified in symfony/ai
The current problem is twofold:

Incorrect Model Catalog: The ModelCatalog (likely for the Google/VertexAI bridge) appears to list Capability::OUTPUT_STRUCTURED for Gemini 2.0 models. This is in direct contradiction with Google's official documentation.

Lack of Exception: The most problematic part is that the AgentProcessor (or a similar class) does not throw any exception (e.g., LogicException or UnsupportedPlatformException) when a capability (OUTPUT_STRUCTURED) is requested but not actually supported by the model. This check seems to have been removed (see this comment in #728). The call fails silently or in an unexpected way, which makes diagnosis and debugging very difficult.

Suggested Improvements
To fix this behavior and improve the component's robustness, I suggest the following actions:

Update the Catalog: Remove Capability::OUTPUT_STRUCTURED from all Gemini models with a version lower than 2.5 in the relevant ModelCatalog files (Google, VertexAI, etc.).

Restore/Add an Exception: Add a check back into the AgentProcessor (or wherever the options are validated) to throw a clear exception if the user tries to use an option (like output_structure) that is not declared in the selected model's capabilities ($model->getCapabilities()).

This would provide "fail-fast" behavior and immediately inform the developer of the incompatibility between their request and the chosen model.

Thank you for your work on this component!

[OskarStark]

Thanks for pointing this out. PR welcome to correct the capabilities 👍🏻