FallbackModel type annotations does not accept string as model leading to type checking issues

[vikigenius]

Initial Checks

• I confirm that I'm using the latest version of Pydantic AI
• I confirm that I searched for my issue in https://github.com/pydantic/pydantic-ai/issues before opening this issue

Description

The Agent __init__ function and many other places now accept Model | KnownModelName | str

But the FallbackModel just accepts Model | KnownModelName it should also accept str

Example Code

from pydantic_ai.models.fallback import FallbackModel

model = FallbackModel('gpt-5', 'gpt-4o') # This would cause a type checker error until `gpt-5` was added to KnownModelName

Python, Pydantic AI & LLM client version

python 3.12
pydantic_ai 0.7.1