…` and `--remove-assignee` (#11056)

* Document support for `@copilot` in `gh [pr|issue] edit --add-assignee` and `--remove-assignee`

Following up on #10991, this updates the help text for
`issue edit` and `pr edit`'s `--add-assignee` and
`--remove-assignee` options to mention that you can use
`@copilot`.

This is already mentioned in the command-level help text, but
not at the argument level, whereas the `@me` macro is.

* Apply suggestion from @babakks

Co-authored-by: Babak K. Shandiz <babakks@github.com>

* Apply suggestion from @babakks

Co-authored-by: Babak K. Shandiz <babakks@github.com>

---------

Co-authored-by: Babak K. Shandiz <babakks@github.com>

Fix pr edit when URL is provided

Signed-off-by: Babak K. Shandiz <babakks@github.com>

Fix const in PR finder tests