fix: minor hookcmds fixes

[tonyandrewmeyer]

Small fixes for the hookcmds package:

• When flattening action parameters, recurse into any mapping, not just mutable mappings (this leaves the model version unchanged, but that's more specifically expecting a dict)
• Correct the return type of action-get, which can be any action parameter type, which means essentially Any
• Correct the docstring of relation-get's unit argument

These were reported by @carlcsaposs-canonical , thanks!

[carlcsaposs-canonical]

I believe None will get data for the unit the charm code is running on, not necessarily the unit that triggered the current event

[carlcsaposs-canonical]

nvm I was mistaken

(think I got mixed up by units having the same data in the databag)

[dimaqq]

Let's validate Carl's comment in real juju.

[james-garner-canonical]

Sorry to revisit a discussion we've had before, but I really don't think the 'cast from json.loads then return' pattern is beneficial here:

• We're now returning an Any typed object in one branch (the result of json.loads(stdout)).
• All other types, including dict[str, Any] are logically subtypes of it.
• The previous cast to two disjoint types was unnecessary due to the immediate return, but casting to a subtype feels somehow even more unnecessary.

Additionally:

• The overloads already capture the return type logic of this function clearly and concisely, and that's all the user will see.
• Most importantly, the conditional cast complexity for readers and maintainers to an otherwise very short and clear function -- for example the changes in this PR are really only to the return type annotation, but require touching the internal if ... else expression too without making any actual runtime changes.

Suggested change

	result = cast('dict[str, Any]', json.loads(stdout)) if key is None else json.loads(stdout)
	return result
	return json.loads(stdout)

If we're really committed to using cast internally to re-document return types, how about lifting the actual evaluation of the result out of the cast logic:

    result = json.loads(stdout)
    if key is None:
        return cast('dict[str, Any]', result)
    return result

[tonyandrewmeyer]

Sorry to revisit a discussion we've had before

I do feel like this is shoe-horning in a previously-resolved discussion into a PR that is mostly unrelated.

I'm ok with re-opening this discussion, but I'd rather that was done through opening an issue or PR that addresses this throughout all of hookcmds (or alternatively, adding to the Charm Tech daily discussions).

[james-garner-canonical]

My rationale for revisiting it here is that even if we think it's the right choice everywhere else in hookcmds I think the changes introduced in this PR make it the wrong choice here, and I'd prefer it not to be committed as-is. But I approved because I do think it's valid call to want to keep it this way for consistency and a smaller diff.

[tonyandrewmeyer]

Yes, which is why I'm leaving it in place. One of the primary reasons that hookcmds is like this is because there was a desire / request to have all of the methods consistent in this area, so I don't want to break that consistency in this PR.