fix(pydantic): improve error message for untyped state parameter

[majiayu000]

Summary

• Fixed uninformative KeyError when pydantic_action has untyped state parameter
• Now raises clear ValueError explaining the state parameter must be annotated

Changes

• Changed type_hints["state"] to type_hints.get("state") to avoid KeyError
• Updated error message to clearly state: "'state' parameter must be annotated with a type extending pydantic.BaseModel"

Test plan

• Added test function _fn_with_untyped_state_arg and parameterized test case
• Added test__validate_and_extract_signature_types_untyped_state_error_message to verify error message
• All existing pydantic tests pass (32 tests)
• Pre-commit checks pass

Closes #385

[skrawcz]

can you show a before and after example here to help understand the change here please?

[skrawcz]

looks good. would love a before and after to show case it.

[majiayu000]

Before and After Example

Before (without this fix)

from burr.integrations.pydantic import pydantic_action

@pydantic_action
def my_action(state):  # missing type annotation
    return state

Error:

KeyError: 'state'

This error is cryptic and doesn't tell the user what went wrong.

---

After (with this fix)

from burr.integrations.pydantic import pydantic_action

@pydantic_action
def my_action(state):  # missing type annotation
    return state

Error:

ValueError: The 'state' parameter must be annotated with a type extending pydantic.BaseModel.

Now the error message clearly explains what the user needs to fix.

[skrawcz]

Great thanks!