feat!: add error if meta not passed when config or actions are in ops.testing

[PietroPasotti]

Currently, if meta isn't passed to Context, and the metadata isn't auto-loaded, a default meta of {'name': '<charm-name>'} is set. This can be confusing to users if they pass in config or actions (which prevents auto-loading) and expect the meta to be auto-loaded.

This PR changes he behaviour so that if actions or config are passed in, then meta must be passed in as well.

Fixes #1492

[tonyandrewmeyer]

What we lose here is being able to do simple contexts like:

ctx = testing.Context(MyCharm, actions={'foo': {'description': '...'}})

Although this isn't that much longer:

ctx = testing.Context(MyCharm, meta={'name': '...'}, actions={'foo': {'description': '...'}})

I think I'm ok with the backwards compatibility breakage - I think generally people are autoloading so this is unlikely to break for anyone. It doesn't seem like there's a backwards compatible option outside of docs or warnings.

An alternative to this would be that if meta/config/actions was None then we could attempt autoloading that specific piece. That does get a bit weird if they're all in charmcraft.yaml, though. Basically, meta/config/actions would be an override over the autoload, rather than an alternative. I think this is what the OP was expecting?

I wonder if we're going to have some backwards compatibility breakage if it would be worth doing more and trying to solve the single-YAML issue too (#1424).

[tonyandrewmeyer]

Suggested change

	# def test_foo():
	# ctx = Context(MyCharm, config={"foo": "bar"})
	# ctx.run(..., State())

I'm assuming this is leftover rather than meant as an example?

[tonyandrewmeyer]

I suppose if we're improving error messages we ought to avoid Scenario as a proper noun 😞

Suggested change

	f"Cannot automatically setup scenario with `charm_type`={charm_type}. "
	"This is likely because this charm has some nonstandard repository setup and Scenario "
	"can't find the charmcraft|(metadata|config|actions) yamls in their usual location. "
	"Please parse and provide their contents manually via the `meta`, `config` "
	"and `actions` Context parameters.",
	f"Cannot automatically set up with `charm_type`={charm_type}. "
	"This is likely because this charm has some non-standard repository setup and the "
	"charmcraft|(metadata|config|actions) YAML files aren't in their usual location. "
	"Please parse and provide their contents manually via the `meta`, `config` "
	"and `actions` Context parameters.",

[tonyandrewmeyer]

And I suppose here also 😞

Suggested change

	raise ContextSetupError(
	"Scenario doesn't support overriding `actions|config` without overriding `meta` too."
	)
	raise ContextSetupError(
	"To override `actions|config` you must also override `meta`."
	)

[tonyandrewmeyer]

I found this initially confusing because there's metadata two lines above, maybe this would be clearer?

Suggested change

	# metadata not found:
	# metadata not found in YAML:

[benhoyt]

I think this is reasonable. Re Tony's comment:

Basically, meta/config/actions would be an override over the autoload, rather than an alternative. I think this is what the OP was expecting?

That sounds too magical to me. I think it should be one or the other, not an override of (some) parts -- I think that would just be confusing.