Search: case modifiers fail on empty or short regex captures

Search view case modifiers mishandle empty or short numbered capture groups. The equivalent replacement in the editor's find widget works correctly.

- VS Code version: 1.136.1
- OS: macOS 27.0, arm64
- Also reproduced directly against the core search replacement implementation at current `main`, `fc0a9e94576224c89cc08d390b38ab760a261f1f`.

Extension isolation: reproduced in core replacement tests without loading extensions. The installed-app UI reproduction below was with extensions enabled; a separate UI run with all extensions disabled has not been verified.

### Reproduction: optional capture

1. Open a folder containing a saved text file with these lines:

```text
user_name
name
id
```

2. Open **Replace in Files** in the Search view (not the editor's single-file find widget).
3. Enable **Use Regular Expression**. Leave **Preserve Case** off.
4. Search for `^(?:([a-z]+)_)?([a-z]+)$`.
5. Replace with `\u$1\u$2`.
6. Inspect the replacement preview, then run Replace All.

Expected saved contents:

```text
UserName
Name
Id
```

Actual preview and saved contents:

```text
UserName
\uName
\uId
```

The first group exists but does not participate in the matches for `name` and `id`; its case modifier should not become literal output.

### Related boundary case: stacked modifiers on a short capture

Search `^([a-z]{1,3})$` and replace with `\u\u\u$1` on separate lines containing `url`, `id`, and `x`. The expected results are `URL`, `ID`, and `X`. In the current core search replacement implementation, `id` and `x` throw `TypeError: Cannot read properties of undefined (reading 'toUpperCase')` because case operations continue past the end of the capture. The editor's find/replace implementation handles these inputs.

### Related reports checked

- #19740 concerns editor find/replace inserting literal `undefined` for an unmatched group without case modifiers. Here, Search view retains the case modifier itself, and editor find/replace produces the expected result.
- #140734 concerns newlines being dropped from multiline replacements using case modifiers. These examples use single-line matches and empty or short captures; the existing newline regression test passes.
- #150070, which was referred to #128134, concerns replacements using lookahead and newline matching with `$1$2$2`. That replacement has no case modifiers and takes the unchanged replacement path.
- #288968 adds named capture group support. These examples use numbered groups; that PR retains the empty-capture check and unbounded case-operation loop involved here.

Workspace search case-modifier support was introduced in #105101.

A small fix with six regression tests is prepared on https://github.com/Suzu1Dev/vscode/tree/fix/search-replace-short-captures. The fixed build passes the same preview and actual Replace All scenarios, 257 related Electron unit tests (3 existing skips), and 83 search integration tests.