Add output_json_schema method to Agent class

[g-eoj]

This PR adds a method to get the json schema for agent output.

It aims to support all output types, including special ones:

• text only
• BinaryImage
• DeferredToolRequests

Simple example:

agent = Agent('test')
print(agent.output_json_schema())
print(agent.output_json_schema(output_type=bool))

{'type': 'string'}
{'type': 'object', 'properties': {'result': {'anyOf': [{'type': 'object', 'properties': {'kind': {'type': 'string', 'const': 'final_result'}, 'data': {'properties': {'response': {'type': 'boolean'}}, 'required': ['response'], 'type': 'object'}}, 'required': ['kind', 'data'], 'additionalProperties': False, 'title': 'final_result'}]}}, 'required': ['result'], 'additionalProperties': False}

Closes #3225

[g-eoj]

@DouweM is there a source of truth I can reference for what the schema output should be for tests? I think I will get stuck figuring this out so any guidance will be appreciated.

Edit: I added snapshots of expected output to the tests. Please review them carefully as I'm guessing what they are supposed to be.

[DouweM]

Why did we need to change this?

[g-eoj]

I modify UnionOutputProcessor so that toolset output processors can be unioned with additional output types (like str, BinaryImage, DeferredToolRequests). Maybe I shouldn't be using toolset.processors?

[g-eoj]

@DouweM thanks for the reviews so far.

The PR has an issue where it always adds str to the output schema if the AutoOutputSchema is selected. However I think this is due to a bug not caused by the PR. I think it is because

pydantic-ai/pydantic_ai_slim/pydantic_ai/_output.py

Line 398 in bcd3d83

text_processor=processor,

will always set allows_text to True. This doesn't seem right but I'll wait for confirmation.