Closes #22473.

Some language servers report a syntax error whose range starts at the
end
of one line and ends at the start of the next — e.g.
Pyright/ruff/ty/pyrefly
on a Python function missing its trailing `:`:

```
range: { start: { line: 0, character: 11 }, end: { line: 1, character: 0 } }
```

That range spans nothing but the line terminator, so there's no glyph
left
for the editor to underline. The diagnostic is real (it shows up in the
status bar and the diagnostics panel) but never gets a visible squiggle
in
the buffer, which makes it very easy to miss — especially since a
missing
`:` is one of the most common Python typos.

## Fix

`update_buffer_diagnostics` already collapses *empty* ranges onto an
adjacent character. This adds a sibling case for ranges that are exactly
`line_start..next_line_start` (i.e. cover only the terminator): they're
pulled back onto the last character of the line they actually concern,
the same way empty ranges are. Ranges are only touched when the end
position is precisely the start of the next line, so genuinely
multi-line
diagnostics that happen to *start* at an end-of-line position are left
untouched.

## Testing

Added `test_diagnostic_range_spanning_line_terminator` in
`project_tests.rs`, modeled on the neighboring
`test_empty_diagnostic_ranges`,
asserting the closing `)` of `def main()` gets the error highlight
instead
of the range being invisible.

Verified against real language servers: reproduced the exact same
`start..next_line_start` range on ruff 0.16.4, ty 0.0.74, and pyrefly
1.2.0
for a missing `:`, matching the LSP trace pyright produced in #22473.

🤖 Generated with [Claude Code](https://claude.com/claude-code)


Release Notes:

- Fixed diagnostics not being underlined when their range covers only a
line terminator (e.g. a missing `:` in Python)

---------

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>
Co-authored-by: Kirill Bulatov <kirill@zed.dev>
Co-authored-by: Kirill Bulatov <mail4score@gmail.com>