Closes #63301

The completion label row set `min_w_0()`, which removes a flex item's
automatic minimum size and lets it shrink below its own content width.
The label inside is `flex_none` and never shrinks, so for long
completion labels the text overflowed the row and, with GPUI's default
`Overflow::Visible`, painted directly on top of the right-aligned
documentation text.

Removing `min_w_0()` restores the row's automatic minimum so it no
longer shrinks past the label. The documentation label already has
`flex_shrink(1.0)`, `min_w_0()` and `truncate()`, so it is now the only
element that gives way, shrinking and ellipsizing as intended.

The resulting layout is `[full label] [documentation, ellipsized when it
overflows]`.

### Before

<img width="687" height="426" alt="Screenshot From 2026-09-02 00-48-03"
src="https://github.com/user-attachments/assets/b734b632-fd05-4afc-8b02-b231922646ef"
/>


### After

<img width="853" height="526" alt="Screenshot From 2026-09-02 02-23-41"
src="https://github.com/user-attachments/assets/60281115-5a8d-4f4e-bfe0-c037ff7244e3"
/>


Release Notes:

- Fixed completion menu labels overlapping the documentation text for
long completion items