Add support for structured outputs in AI agent

[Hooghof]

As a user, I want to configure a JSON schema that constrains the Agent's output, so that I can reliably use the Agent's responses further in the flow

Acceptance criteria

1. I can add a JSON schema to a structured output configuration, either inline via a text box or via a reference to a header.
2. Upon saving the configuration the JSON schema is validated and I get a clear message if something is wrong
3. The output of the agent is enforced according to the schema
4. If the agent was unable to format the output according to the schema it retries up to a predefined number of attempts and ultimately fails with a clear message and triggers the error route.

In the LangChan4j vocabulary this is referred to as the jsonSchema endpoint option

Jira: https://dovetailworld.atlassian.net/browse/FG-2635