Long option completion with = broken #2847

When using Click shell completion in bash and zsh, long option completion is not working properly when = is used to separate the option from the value (e.g., command --long-option=value vs. command --long-option value). In zsh, the option being completed is replaced with the incomplete value or matched value, essentially "gobbling" up the option. In bash, the option isn't completed at all. I did not test fish.

Steps to reproduce the issue

1. Create the following script and place it in the bin directory of an activated virtual environment with click installed:

# gobble

#!/usr/bin/env python
import click


@click.command
@click.option("--color", type=click.Choice(["auto", "always", "never"]), default=None,
              help="Control colors in output.")
@click.option("--name",
              help="Name of something.")
def main(color: str | None, name: str | None) -> None:
    """Test long options."""
    click.echo(f"{color=}, {name=}")


if __name__ == "__main__":
    main()

2. Open a zsh shell and enable script completion: eval "$(_GOBBLE_COMPLETE=zsh_source gobble)"

3. Attempt to tab complete the --color option using both styles

4. Open a bash shell and enable script completion: eval "$(_GOBBLE_COMPLETE=bash_source gobble)"

5. Attempt to tab complete the --color option using both styles

Expected behavior

Environment:

• Python version: 3.12.7
• Click version: 8.1.8