Clarify usage of agent factories

[dsfaccini]

Clarify that agents can be produced by a factory function if preferred.

[DouweM]

Where does the factory function idea come from? I don't think we need to mention that specifically. Going by https://pydanticlogfire.slack.com/archives/C083V7PMHHA/p1763775638553099, we mostly need to make it clear that it's totally fine to create an agent dynamically at any point of your application.

[dsfaccini]

factory function + example seemed the clearest wording to me to communicate that you can also make them dynamically

[dsfaccini]

be created dynamically, for example, by a factory function like `make_agent('agent-type')`, whichever you prefer

[DouweM]

@dsfaccini I think that's still very prescriptive, as dynamic can also just mean you can build it inside a FastAPI endpoint function on the fly, no factory function needed. The intent of this change is too loosen our wording to be less prescriptive and basically say "whatever you want is fine", not to change "do it this way" to "do it this way or that way" when there are many other possible ways

[dsfaccini]

!!! tip "Agents are designed for reuse, like FastAPI Apps"
    You can instantiate one agent and use it globally throughout your application, as you would a small [FastAPI][fastapi.FastAPI] app or an [APIRouter][fastapi.APIRouter], or dynamically create as many agents as you want. Both are valid and supported ways to use agents.