Mark instance as non-Optional

[florimondmanca]

What does this PR do?

Convince IDEs and type checkers that self.instance always refers to a well-defined instance dict, although it is not technically enforced at runtime yet.

Motivation

Currently, mypy always complains (as it should) when we're trying to access anything on self.instance, because it's marked as Optional.

This is technically correct, because some tests still use the legacy signature without passing an instance.

But in practice (i.e. when an Agent is running a check in the wild) the instance is always set, so it makes better sense to mark it as a non-optional dict.

Additional Notes

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have changelog/ and integration/ labels attached

[codecov]

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