test: run assembled setup harness scripts from a temp file, not bash -c argv (Windows MSYS2 8 KB truncation)

windows-free-tests (run 33907177851) failed in
test/setup-alias-name-uniqueness.test.ts with
  bash: -c: line 178: unexpected EOF while looking for matching `'
The harness slices functions out of `setup` and passed the joined script as
one `bash -c` argv element. The ownership gate grew that script from 6.7 KB
to 15.7 KB, and on Windows bash is an MSYS2 program: when its parent is a
non-MSYS process (bun), msys-2.0.dll's build_argv() runs any argument
containing `?*["'(){}` through globify()/glob(), which copies the pattern
into a fixed `Char patbuf[8192]` and silently stops after 8192 - MB_CUR_MAX
(8186 chars under C.UTF-8); GLOB_NOCHECK then returns the truncated text as
the argument. Character 8186 lands inside the single-quoted sed token on
line 178. Rebuilding the exact script with CI path shapes and cutting it at
8186-8190 characters reproduces the identical message locally; cmd.exe's
8191-UTF-16 cap and CreateProcess's 32767 do not fit the evidence.

Fix: test/helpers/bash-script.ts writes the script to a temp file and runs
`bash <path>` — a short glob-free argument that never enters globify. Every
setup harness that assembled a script for `bash -c` (11 files, 22 sites)
uses it; timeouts and env are preserved verbatim, spawn/timeout errors are
appended to stderr, temp cleanup is best-effort. `spawnSync('bash',
[<Windows absolute path>])` already passes on windows-latest in setup-help,
uninstall-windows-copies and the migration tests. The Windows-curated list
is byte-identical before and after.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

test/setup-windows-fallback.test.ts

@@ -1,5 +1,5 @@
 import { describe, test, expect } from 'bun:test';
-import { spawnSync } from 'child_process';
+import { runBashScript } from './helpers/bash-script';
 import * as path from 'path';
 import * as fs from 'fs';
 import * as os from 'os';
@@ -95,10 +95,7 @@ describe.skipIf(process.platform === 'win32')('setup: _link_or_copy helper — b
       const helper = extractHelper();
       // IS_WINDOWS must exist as a shell-readable var before sourcing.
       const script = `IS_WINDOWS=${isWindows}\n${helper}\n_link_or_copy "${src}" "${dst}"\n`;
-      const result = spawnSync('bash', ['-c', script], {
-        encoding: 'utf-8',
-        timeout: 5000,
-      });
+      const result = runBashScript(script, { timeout: 5000 });
       const lst = fs.lstatSync(dst, { throwIfNoEntry: false });
       return {
         ok: result.status === 0,