Fix enum convertor returning None for already-converted values

[bysiber]

Summary

generate_enum_convertor silently returns None when it receives a value that's already an enum member. This happens when an enum parameter has a user-defined callback — the value gets double-converted.

The Bug

The enum convertor builds a lookup map from str(val.value) for each member, then looks up incoming values using str(value). When the value is already an enum member (e.g., Network.conv), str(Network.conv) returns "Network.conv" — which doesn't match any key in val_map (which contains "conv"). The convertor falls through and implicitly returns None.

This happens because the parameter callback wrapper (via get_param_callback) converts the string to an enum, then the command callback wrapper (via get_callback) runs the convertor again on the already-converted value.

from enum import Enum
from typing import Annotated
import typer

class Network(Enum):
    simple = "simple"
    conv = "conv"

app = typer.Typer()

def validate(value: Network) -> Network:
    return value

@app.command()
def main(network: Annotated[Network, typer.Option(callback=validate)] = Network.simple):
    print(f"Got: {network!r}")  # Prints: Got: None

app()

The Fix

Added an isinstance guard to return the value as-is when it's already an enum member — the same pattern used by param_path_convertor a few lines above.

[svlandeg]

Hi, thanks for the PR! Could you look into the failing CI? I'll put this in draft in the meantime 🙏

[bysiber]

Added a test for the newly covered branch — it checks that passing an already-converted enum member through the convertor returns it unchanged. CI should be green now.