Move fix suggestion to subdiagnostic (#19464)

Summary
--

This PR tweaks Ruff's internal usage of the new diagnostic model to more
closely
match the intended use, as I understand it. Specifically, it moves the
fix/help
suggestion from the primary annotation's message to a subdiagnostic. In
turn, it
adds the secondary/noqa code as the new primary annotation message. As
shown in
the new `ruff_db` tests, this more closely mirrors Ruff's current
diagnostic
output.

I also added `Severity::Help` to render the fix suggestion with a
`help:` prefix
instead of `info:`.

These changes don't have any external impact now but should help a bit
with #19415.

Test Plan
--

New full output format tests in `ruff_db`

Rendered Diagnostics
--

Full diagnostic output from `annotate-snippets` in this PR:

``` 
error[unused-import]: `os` imported but unused
  --> fib.py:1:8
   |
 1 | import os
   |        ^^
   |
 help: Remove unused import: `os`
```

Current Ruff output for the same code:

```
fib.py:1:8: F401 [*] `os` imported but unused
  |
1 | import os
  |        ^^ F401
  |
  = help: Remove unused import: `os`
```

Proposed final output after #19415:

``` 
F401 [*] `os` imported but unused
  --> fib.py:1:8
   |
 1 | import os
   |        ^^
   |
 help: Remove unused import: `os`
```

These are slightly updated from
https://github.com/astral-sh/ruff/pull/19464#issuecomment-3097377634
below to remove the extra noqa codes in the primary annotation messages
for the first and third cases.

crates/ruff_linter/src/test.rs

@@ -272,7 +272,7 @@ Either ensure you always emit a fix or change `Violation::FIX_AVAILABILITY` to e
             }
 
             assert!(
-                !(fixable && diagnostic.suggestion().is_none()),
+                !(fixable && diagnostic.first_help_text().is_none()),
                 "Diagnostic emitted by {rule:?} is fixable but \
                 `Violation::fix_title` returns `None`"
             );