input: Allow configuring the completion popover max width (#2369)

## Summary

The LSP completion popover width is currently capped at a hardcoded
`MAX_MENU_WIDTH` of 320 px, and there's no public API to override it
from outside the crate. That's fine for most identifiers, but it
truncates longer LSP labels — e.g. database table identifiers like
`tbl_verification_tracking` that show up in completions for editors
hosting SQL/MongoDB/etc. clients.

This PR keeps the default at **320 px** (no visual change for existing
callers) and adds a way to widen the popover per editor:

- Renames the hardcoded `MAX_MENU_WIDTH` to `DEFAULT_MAX_MENU_WIDTH`
(still `pub(crate)`).
- Stores `max_width: Pixels` on `CompletionMenu` and threads it through
`CompletionMenu::new`, the items column, the documentation panel, and
the side-by-side vs vertical layout overflow check.
- Adds two public methods on `InputState`:
  - `completion_menu_max_width(width: Pixels) -> Self` — builder.
- `set_completion_menu_max_width(width, window, cx)` — runtime setter
that also forwards into an already-open popover.

Usage:

```rust
InputState::new(window, cx)
    .code_editor("sql")
    .completion_menu_max_width(px(480.))
```

## AI Assistance

🤖 The patch was drafted by Claude and reviewed/tested by a human; it
follows the existing builder-method pattern used for `placeholder`,
`searchable`, `folding`, etc.

## Test plan

- [x] `cargo check -p gpui-component`
- [x] `cargo clippy -p gpui-component -- -D warnings`
- [x] `rustfmt` on the three touched files
- [ ] Manual: open an editor with long completion labels, verify the
popover caps at the configured width and still shrinks to fit when near
the right viewport edge.
- [ ] Manual: confirm default behavior (no builder call) is unchanged at
320 px.

---------

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Co-authored-by: Jason Lee <huacnlee@gmail.com>