Respect the `filterText` of LSP completion items during completion filtering (#62433)

# Objective

Closes #61646.

For code completions, Zed currently fuzzy-matches against
`CodeLabel::filter_text()`:

https://github.com/zed-industries/zed/blob/d4010e91ccc9635853698af160f15bb479a484d0/crates/editor/src/code_context_menus.rs#L337-L343

`CodeLabel::filter_text()` is a substring of `CodeLabel.text`, which is
essentially the text itself. `CodeLabel.text` is constructed by Zed's
per-language adapters from the `label` and `detail` fields of the
completion items returned by the language server — the exact
construction differs from adapter to adapter, but the source data is the
same. In effect, `CodeLabel.text` ≈ `label` + `detail`. Zed therefore
filters on the server-returned `label` and `detail`, while the
server-returned `filterText` field is silently ignored.Per the LSP spec:
```
	/**
	 * A string that should be used when filtering a set of
	 * completion items. When omitted, the label is used as the
	 * filter text for this item.
	 */
	filterText?: string;
```
we should use `filterText` when it is provided.

Normally, language servers populate `filterText` as a substring of
`label`, so the current behavior works fine. But for certain language
servers or functions, `filterText` can be entirely unrelated to `label`
and `detail`. For example, for `std::path::Path::parent()` in Rust,
rust-analyzer returns:
```json
{
        "label": "parent()",
        "labelDetails": {
          "detail": "(alias dirname)",
          "description": "fn(&self) -> Option<&Path>"
        },
        "kind": 2,
        "preselect": true,
        "sortText": "7ffffff6",
        "filterText": "parentdirname",
        ...
}
```
Typing `dirname` therefore never surfaces this completion.

The root design issue behind this bug is that `CodeLabel` is not
well-suited to filtering LSP completions.

## Solution

`CodeLabel` and its related methods are kept untouched: the struct is
reused across the repo and is only unsuitable for filtering LSP
completions. Instead, the changes are made in `CompletionSource` and
`Completion`, each gaining a `filter_text()` method:

- `CompletionSource::filter_text()` handles LSP completions, returning
the server-provided `filterText` and falling back to the `label` when
`filterText` is absent.
- `Completion::filter_text()` is the general entry point used for fuzzy
matching; for non-LSP completions it falls back to the existing
`label.filter_text()`.

The fuzzy match target is switched from `CodeLabel::filter_text()` to
`Completion::filter_text()` — that is the core change.

Since the fuzzy match target is no longer guaranteed to be a substring
of the displayed `CodeLabel.text`, the matched characters no longer have
a direct position in the displayed text to highlight. Bold highlights
are therefore only rendered when `CodeLabel::filter_text()` equals
`Completion::filter_text()`. This is a safe choice, though not an ideal
one.

## Testing

Added a new GPUI test covering the new behavior; also built and tested
with a before/after comparison, attached in the Showcase section.

## 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 adheres to Zed's UI standards
([UX/UI](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
and
[icon](https://github.com/zed-industries/zed/blob/main/crates/icons/README.md)
guidelines)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

## Showcase

| Before | After |
|:--:|:--:|
| <img width="708" height="308" alt="Before"
src="https://github.com/user-attachments/assets/ca2e3820-7ea1-4dcc-a91f-28aab71aecc5"
/> | <img width="696" height="248" alt="After"
src="https://github.com/user-attachments/assets/334e240b-64d5-495b-aef6-772456b993ba"
/> |

---

Release Notes:

- Improved completion filtering for lsp completions.