feat: Add support for agent invoke with no input, or Message input

[Unshure]

Description

Allow an agent to be invoked with no input, or a list of messages

This updates the agents execution to support additional invocation scenarios:

First, you can invoke an agent with just the current state of the messages array:

agent = Agent(messages=[{"role":"user", "content":[{"text":"Hello!"}]}])
agent()

Additionally, you can invoke the agent with an input list of messages:

agent = Agent()
agent([
  {"role":"user", "content":[{"text":"Hello!"}]},
  {"role":"assistant", "content":[{"text":"How are"}]},
])

This helps enable things like "putting words in claudes mouth".

Related Issues

N/A

Documentation PR

N/A

Type of Change

New feature

Testing

How have you tested the change? Verify that the changes do not break functionality or introduce warnings in consuming repositories: agents-docs, agents-tools, agents-cli

• I ran hatch run prepare

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

---

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[mkmeral]

with this PR, what's the desired state when calling with None?

[dbschmigelski]

What are your thoughts on turning str | list[ContentBlock] | Messages | None into a type alias? Seems like we're copying it around and it might be helpful to have some Prompt (don't over index on the name) type

[dbschmigelski]

Unlikely to be breaking anyone because of

def get_tracer() -> Tracer:
    """Get or create the global tracer.

    Returns:
        The global tracer instance.
    """
    global _tracer_instance

    if not _tracer_instance:
        _tracer_instance = Tracer()

    return _tracer_instance

?