## Summary

When formatting specific ranges (e.g. via Format Selections), trailing
whitespace removal and final newline enforcement currently operate on
the entire file. This PR makes these operations range-aware, so they
only affect lines within the target ranges.

As noted in #16509, users want `remove_trailing_whitespace_on_save` and
`ensure_final_newline_on_save` to suppress jitter by only touching
changed lines. This PR provides the foundational infrastructure for that
— range-based whitespace and newline operations — which a follow-up PR
will wire into the `format_on_save` modifications mode.

## Changes

- Added `remove_trailing_whitespace_in_ranges` and
`ensure_final_newline_in_range` to `Buffer`
- When a `FormattableBuffer` has ranges set, whitespace and newline
operations now use the range-based variants
- `format_ranges_via_lsp` returns `Ok(None)` instead of an error when
the LSP doesn't support range formatting
- Extracted `anchor_ranges_to_row_ranges` helper to deduplicate
conversion logic

<details><summary>Tests (4)</summary>

- `test_trailing_whitespace_in_ranges` — only modified lines have
whitespace removed
- `test_trailing_whitespace_empty_ranges` — empty ranges → no changes
- `test_final_newline_modified_last_line` — newline added when last line
is in range
- `test_final_newline_unmodified_last_line` — newline not added when
last line is outside range

</details>

---

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- Improved Format Selection to scope trailing whitespace removal and
final newline enforcement to the selected range instead of the entire
file
- Improved handling of LSP servers that do not support range formatting
by gracefully skipping instead of producing an error

---------

Co-authored-by: Kirill Bulatov <kirill@zed.dev>