## Summary
This PR proposes that we add a new `set_concise_message` functionality
to our `Diagnostic` construction API. When used, the concise message
that is otherwise auto-generated from the main diagnostic message and
the primary annotation will be overwritten with the custom message.
To understand why this is desirable, let's look at the `invalid-key`
diagnostic. This is how I *want* the full diagnostic to look like:
<img width="620" height="282" alt="image"
src="https://github.com/user-attachments/assets/3bf70f52-9d9f-4817-bc16-fb0ebf7c2113"
/>
However, without the change in this PR, the concise message would have
the following form:
```
error[invalid-key]: Unknown key "Age" for TypedDict `Person`: Unknown key "Age" - did you mean "age"?
```
This duplication is why the full `invalid-key` diagnostic used a main
diagnostic message that is only "Invalid key for TypedDict `Person`", to
make that bearable:
```
error[invalid-key] Invalid key for TypedDict `Person`: Unknown key "Age" - did you mean "age"?
```
This is still less than ideal, *and* we had to make the "full"
diagnostic worse. With the new API here, we have to make no such
compromises. We need to do slightly more work (provide one additional
custom-designed message), but we get to keep the "full" diagnostic that
we actually want, and we can make the concise message more terse and
readable:
```
error[invalid-key] Unknown key "Age" for TypedDict `Person` - did you mean "age"?
```
Similar problems exist for other diagnostics as well (I really want this
for https://github.com/astral-sh/ruff/pull/21476). In this PR, I only
changed `invalid-key` and `type-assertion-failure`.
The PR here is somewhat related to the discussion in
https://github.com/astral-sh/ty/issues/1418, but note that we are
solving a problem that is unrelated to sub-diagnostics.
## Test Plan
Updated tests
7a739d6b76