Fix AttributeError crash in Argument.dest on invalid option strings

[EternalRights]

Fix AttributeError crash in Argument.dest on invalid option strings

When pytest_addoption registers an option name that argparse rejects
(e.g. one starting with a digit), Action.init fails during
_set_opt_strings before self.dest is set. If Argument.dest then
reads self._action.dest, the secondary AttributeError hides the
original argparse error.

Fixed by using getattr(self._action, "dest", "") in the
dest property so a missing dest attribute does not cause a second
crash.

This complements #14429 by completing the fix RonnyPfannschmidt
originally requested on #13817.

Closes #13817

[The-Compiler]

I'm confused by this, wasn't #13817 already fixed in #14429? I can't reproduce my example anymore, and your PR description ("When pytest_addoption registers an option name that argparse rejects
(e.g. one starting with a digit)") seems to say something entirely different to your test case.

[EternalRights]

You're right — #14429 fixed the crash path. repr no longer touches
self.dest when _action isn't set.

Ronny's original comment was two things: guard repr, and safe
getattr for dest. I only did the first one. This closes the gap.

The test is synthetic because after #14429, repr is the only thing
that ever accesses .dest on a broken Argument. So there's no real crash
path left. But it's one line, and it's what Ronny asked for.

I'll update the description and rename the changelog (13817 is taken
by #14429, will use 14773 instead).

[The-Compiler]

Let's see what Ronny says about this. IMHO having a .dest attribute default to <missing> seems like quite the hack.

[RonnyPfannschmidt]

i need a p practical example of this failure - i never saw this in the wild

[EternalRights]

Took another look — no real crash path. argparse.Action.init sets self.dest first, and add_argument either returns a complete action or raises before Argument is constructed. After #14429 the only path through repr is safe too. I'll close this.