## Summary

Fix `write_source_file` failing with shell syntax errors when paths
contain metacharacters like parentheses.

## Problem

The generated shell script assigned paths to variables without quoting:

```bash
in=$current_working_dir/path/to/(folder)/file.txt
out=path/to/(folder)/file.txt
```

Bash interprets `(` as the start of a subshell, causing:
```
syntax error near unexpected token `('
```

This breaks builds using Next.js App Router route groups (e.g.,
`app/(dashboard)/page.tsx`) or any path with parentheses, spaces, or
other shell metacharacters.

## Fix

Quote the variable assignments:

```bash
in="$current_working_dir/path/to/(folder)/file.txt"
out="path/to/(folder)/file.txt"
```

## Changes

- `lib/private/write_source_file.bzl`: Quote path variable assignments
- `lib/tests/write_source_files/write_source_file_test.bzl`: Quote paths
in test script generator
- Added regression test with parentheses in path
(`(route-group)/test.js`)

## Test plan

- [x] New test `special_chars_in_path_test` passes
- [x] All existing `write_source_files` tests pass

This allows the content produced by these repo rules to be fetched from
the remote repo contents cache.