Render unsupported syntax errors in formatter tests (#20777)

## Summary

Based on the suggestion in
https://github.com/astral-sh/ruff/issues/20774#issuecomment-3383153511,
I added rendering of unsupported syntax errors in our `format` test.

In support of this, I added a `DummyFileResolver` type to `ruff_db` to
pass to `DisplayDiagnostics::new` (first commit). Another option would
obviously be implementing this directly in the fixtures, but we'd have
to import a `NotebookIndex` somehow; either by depending directly on
`ruff_notebook` or re-exporting it from `ruff_db`. I thought it might be
convenient elsewhere to have a dummy resolver, for example in the
parser, where we currently have a separate rendering pipeline
[copied](https://github.com/astral-sh/ruff/blob/main/crates/ruff_python_parser/tests/fixtures.rs#L321)
from our old rendering code in `ruff_linter`. I also briefly tried
implementing a `TestDb` in the formatter since I noticed the
`ruff_python_formatter::db` module, but that was turning into a lot more
code than the dummy resolver.

We could also push this a bit further if we wanted. I didn't add the new
snapshots to the black compatibility tests or to the preview snapshots,
for example. I thought it was kind of noisy enough (and helpful enough)
already, though. We could also use a shorter diagnostic format, but the
full output seems most useful once we accept this initial large batch of
changes.

## Test Plan

I went through the baseline snapshots pretty quickly, but they all
looked reasonable to me, with one exception I noted below. I also tested
that the case from #20774 produces a new unsupported syntax error.

crates/ruff_python_formatter/resources/test/fixtures/ruff/expression/fstring.py

@@ -706,6 +706,8 @@ f'{1:hy "user"}'
 f'{1: abcd "{1}" }'
 f'{1: abcd "{'aa'}" }'
 f'{1=: "abcd {'aa'}}'
+# FIXME(brent) This should not be a syntax error on output. The escaped quotes are in the format
+# spec, which is valid even before 3.12.
 f'{x:a{z:hy "user"}} \'\'\''
 
 # Changing the outer quotes is fine because the format-spec is in a nested expression.