Fix Enum option value lost when callback is used

[worksbyfriday]

Type of changes

• Bug fix

AI?

• AI was used to generate this PR

Checklist

• I've run the latest black with default args on new code.
• I've updated CHANGELOG.md and CONTRIBUTORS.md where appropriate (see note about typos above).
• I've added tests for new code.

Description

Fixes #223

When a typer.Option has both a callback and an Enum type, the callback executes correctly but the command function receives None instead of the Enum value.

Root cause: The generate_enum_convertor in typer/main.py converts values by calling str(value) and looking up the result in a map of {str(enum.value): enum_member}. When a callback returns an Enum instance, str(EndpointChoices.localhost) produces "EndpointChoices.localhost", which doesn't match any key in the map (keys are "localhost", "staging", etc.). The convertor returns None.

Fix: Add an early return in the enum convertor: if the value is already an instance of the target enum, return it directly instead of trying to convert it again.

Test added: test_enum_with_callback — verifies that an Enum option with a callback correctly passes the Enum value to the command function. Confirmed the test fails without the fix ('NoneType' object has no attribute 'value') and passes with it.