AI junk

[Cyriellewu]

Fixes #3652.

Problem

An option declared with multiple=True renders a metavar identical to a single-value option, so --help gives the user no signal that it can be repeated.

Cause

The base Parameter.make_metavar only appends ... when nargs != 1. multiple=True keeps nargs == 1, so the ellipsis was never added.

Fix

Append ... when self.multiple as well:

Options:
  --foo TEXT...  A list of foo strings.
  --bar TEXT     single
  --help         Show this message and exit.
```n
Scoped to the base `Parameter.make_metavar` (used by `Option`); `Argument` has its own override and already uses `nargs=-1` for repetition, so it's untouched.

## Tests

Added `test_multiple_option_metavar_ellipsis`. The full suite passes locally (1870 passed, no regressions); `ruff` is clean. CHANGES.md updated.