Add AgentRun.{all,new}_messages{_json}

[tibbe]

This allows convenient access to these even if the run didn't end, at which point they would not be available via AgentRun.result.

Let me know if you also want the _json versions and/or if you want the documentation to read differently.

[tibbe]

FYI I tried to follow https://github.com/pydantic/pydantic-ai/blob/main/docs/contributing.md but:

• I had to install pyright myself (uv tool install pyright) and
• that caused make to fail with a bunch of unrelated type errors.

Example of pyright errors:

PYRIGHT_PYTHON_IGNORE_WARNINGS=1 uv run pyright 
/Users/tibbe/work/pydantic-ai/examples/pydantic_ai_examples/ag_ui/api/agentic_generative_ui.py
  /Users/tibbe/work/pydantic-ai/examples/pydantic_ai_examples/ag_ui/api/agentic_generative_ui.py:30:5 - error: Type of "steps" is partially unknown
    Type of "steps" is "list[Unknown]" (reportUnknownVariableType)
/Users/tibbe/work/pydantic-ai/examples/pydantic_ai_examples/ag_ui/api/shared_state.py
  /Users/tibbe/work/pydantic-ai/examples/pydantic_ai_examples/ag_ui/api/shared_state.py:64:5 - error: Type of "special_preferences" is partially unknown
    Type of "special_preferences" is "list[Unknown]" (reportUnknownVariableType)
  /Users/tibbe/work/pydantic-ai/examples/pydantic_ai_examples/ag_ui/api/shared_state.py:71:5 - error: Type of "ingredients" is partially unknown
    Type of "ingredients" is "list[Unknown]" (reportUnknownVariableType)
/Users/tibbe/work/pydantic-ai/examples/pydantic_ai_examples/data_analyst.py
  /Users/tibbe/work/pydantic-ai/examples/pydantic_ai_examples/data_analyst.py:12:5 - error: Type of "output" is partially unknown
    Type of "output" is "dict[Unknown, Unknown]" (reportUnknownVariableType)

[tibbe]

Also either my code is wrong or I cannot figure out how to do these snapshot tests correctly.