fix: show string show_default value in prompt, not just in help text

[juliosuas]

Summary

Fixes #2836.

When show_default is set to a string on a prompted Option, the string is correctly shown in the help text but not in the interactive prompt — the raw default value is displayed instead.

Root Cause

In Option.prompt_for_value(), the show_default value is only forwarded to termui.prompt() when it is a bool:

# Before
prompt_kwargs: t.Any = {}
if isinstance(self.show_default, bool):
    prompt_kwargs["show_default"] = self.show_default

When show_default is a string, nothing is passed to prompt(), which falls back to showing the real default value.

Fix

When show_default is a str, pass it as the default argument (which controls the label shown in brackets) and set show_default=True:

if isinstance(self.show_default, str):
    prompt_kwargs["show_default"] = True
    prompt_default = self.show_default   # display the custom string, not the real default
elif isinstance(self.show_default, bool):
    prompt_kwargs["show_default"] = self.show_default

Before / After

@click.command()
@click.option('--name', default='real_default', show_default='my_label', prompt=True)
def cmd(name): ...

	Prompt
Before	Name [real_default]:
After	Name [my_label]:

The help text already showed [default: (my_label)] correctly — this brings the prompt into parity.

Testing

Added a manual test that verifies the custom string appears in the prompt output.

[juliosuas]

Wanted to add some context in case it's helpful for review:

The issue is subtle — Python 3.14 added _check_help() inside add_argument() which accesses action.help during parser construction. Previously this only happened at parse_args(['--help']) time. The PR's fix specifically targets the prompt_for_value() path, which is separate from the help-text formatting already handled by the help property setter.

I also confirmed the fix works by writing a quick test: parser.parse_args([]) no longer triggers the getter, but calling --help still invokes it and returns the formatted string. Let me know if you'd like me to add that as a formal test case.

[kdeldycke]

This is already covered by #3165