feat: enabling output_schema and tools to coexist

[copybara-service]

feat: enabling output_schema and tools to coexist

This CL enables the simultaneous use of output_schema (structured output) and tools for models that do not natively support both features at once (specifically Gemini 1.x and 2.x on Vertex AI).

Core Logic

The CL implements a workaround for models with this limitation:

1. Synthetic Tooling: Instead of passing the output_schema directly to the model's configuration, it introduces a synthetic tool called set_model_response.
2. Schema Injection: The parameters of this tool are set to the requested output_schema.
3. Instruction Prompting: System instructions are appended, directing the model to provide its final response using this specific tool in the required format.
4. Response Interception: The BaseLlmFlow is updated to check if set_model_response was called. If so, it extracts the JSON arguments and converts them into a standard model response event.

Key Changes

• OutputSchema.java (New): A new RequestProcessor that detects when the workaround is needed, adds the SetModelResponseTool, and provides utilities for extracting the structured response.
• SetModelResponseTool.java (New): A marker tool that simply returns its input arguments, used to "capture" the structured output from the model.
• ModelNameUtils.java: Added logic to identify Gemini 1.x and 2.x models and determine if they can handle native output_schema alongside tools.
• BaseLlmFlow.java: Updated the flow logic to detect the synthetic tool response and generate the final output event.
• Basic.java: Updated to prevent native outputSchema configuration when the workaround is active.
• SingleFlow.java: Registered the new OutputSchema processor.