Use new diff rendering format in tests (#20101)
## Summary
I spun this off from #19919 to separate the rendering code change and
snapshot updates from the (much smaller) changes to expose this in the
CLI. I grouped all of the `ruff_linter` snapshot changes in the final
commit in an effort to make this easier to review. The code changes are
in [this
range](https://github.com/astral-sh/ruff/pull/20101/files/619395eb417d2030e31fbb0e487a72dac58902db).
I went through all of the snapshots, albeit fairly quickly, and they all
looked correct to me. In the last few commits I was trying to resolve an
existing issue in the alignment of the line number separator:
https://github.com/astral-sh/ruff/blob/73720c73be981df6f71ce837a67ca1167da0265e/crates/ruff_linter/src/rules/flake8_comprehensions/snapshots/ruff_linter__rules__flake8_comprehensions__tests__C409_C409.py.snap#L87-L89
In the snapshot above on `main`, you can see that a double-digit line
number at the end of the context lines for a snippet was causing a
misalignment with the other separators. That's now resolved. The one
downside is that this can lead to a mismatch with the diagnostic above:
```
C409 [*] Unnecessary list literal passed to `tuple()` (rewrite as a tuple literal)
--> C409.py:4:6
|
2 | t2 = tuple([1, 2])
3 | t3 = tuple((1, 2))
4 | t4 = tuple([
| ______^
5 | | 1,
6 | | 2
7 | | ])
| |__^
8 | t5 = tuple(
9 | (1, 2)
|
help: Rewrite as a tuple literal
1 | t1 = tuple([])
2 | t2 = tuple([1, 2])
3 | t3 = tuple((1, 2))
- t4 = tuple([
4 + t4 = (
5 | 1,
6 | 2
- ])
7 + )
8 | t5 = tuple(
9 | (1, 2)
10 | )
note: This is an unsafe fix and may remove comments or change runtime behavior
```
But I don't think we can avoid that without really reworking this
rendering to make the diagnostic and diff rendering aware of each other.
Anyway, this should only happen in relatively rare cases where the
diagnostic is near a digit boundary and also near a context boundary.
Most of our diagnostics line up nicely.
Another potential downside of the new rendering format is its handling
of long stretches of `+` or `-` lines:
```
help: Replace with `Literal[...] | None`
21 | ...
22 |
23 |
- def func6(arg1: Literal[
- "hello",
- None # Comment 1
- , "world"
- ]):
24 + def func6(arg1: Literal["hello", "world"] | None):
25 | ...
26 |
27 |
note: This is an unsafe fix and may remove comments or change runtime behavior
```
To me it just seems a little hard to tell what's going on with just a
long streak of `-`-prefixed lines. I saw an even more exaggerated
example at some point, but I think this is also fairly rare. Most of the
snapshots seem more like the examples we looked at on Discord with
plenty of `|` lines and pairs of `+` and `-` lines.
## Test Plan
Existing tests plus one new test in `ruff_db` to isolate a line
separator alignment issue
