Append the fix title to a Diagnostic on drop

[ntBre]

Summary

As noted in #22972 (comment),
attaching sub-diagnostics to Ruff's diagnostics can interact a bit awkwardly
with how we attach the Violation::fix_title and then render a fix diff. In
particular, the fix title is currently attached as a help sub-diagnostic when
the diagnostic is originally created, meaning that any additional
sub-diagnostics appear between the fix title and the rendered fix:

RUF064 [*] Non-octal mode
 --> RUF064.py:6:17
  |
4 | from pathlib import Path
5 |
6 | os.chmod("foo", 444)  # Error
  |                 ^^^
7 | os.chmod("foo", 0o444)  # OK
8 | os.chmod("foo", 7777)  # Error
  |
help: Replace with octal literal
info: Current value of mode 444 (0o674) sets permissions: u=rw-, g=rwx, o=r--)
info: Suggested value of 292 sets permissions: u=r--, g=r--, o=r--
3 | import os
4 | from pathlib import Path
5 |
  - os.chmod("foo", 444)  # Error
6 + os.chmod("foo", 0o444)  # Error
7 | os.chmod("foo", 0o444)  # OK
8 | os.chmod("foo", 7777)  # Error
9 | os.chmod("foo", 10000)  # Error
note: This is an unsafe fix and may change runtime behavior

Instead of adding the fix title immediately, this PR stores it on our
DiagnosticGuard type to be added just before the guard is dropped and the
diagnostic is stored.

I think a better long-term fix would be to attach the diff to the sub-diagnostic
with the help message/fix title somehow and render these info
sub-diagnostics after the diff (or let the diagnostic author choose the
order), but this seemed like an easy improvement over the current approach, at
least.

I also remembered that @MichaReiser and I had discussed this a bit before and found the comment. Micha's suggestion was to add something like a Checker::report_custom_diagnostic method that leaves out the fix title, expecting the diagnostic author to attach it later. I also tried this approach but found it a bit awkward, at least with the current version of RUF064. For example, RUF064 has several early returns intermixed with the info additions, so you have to be sure to attach the help in each of those cases. Then I tried to get really cute with a defer method that would run a closure before drop so that you could do something like this:

let mut diagnostic = checker.report_custom_diagnostic(...);
diagnostic.defer(|diag| diag.help(...));

but that also became a bit painful with all of the generic/lifetime changes. Maybe I picked a bad type for the deferred function field, though. Anyway, I think the approach in this PR is a reasonable improvement for now.

Test Plan

Existing tests for ISC004 and from #22972 showing the help message at the end.

[astral-sh-bot]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

[ntBre]

Ahhh, these test failures are showing a case where we actually want the info diagnostic second, so simply attaching the fix title last isn't going to work, even for our few existing sub-diagnostics. I'll close this in favor of #23044.