fix(unix): do not rewrite already-unpacked asar helper paths

[fei (feiiiiii5)]

Summary

Fixes #923. When node-pty is installed under app.asar.unpacked (the layout Electron apps use for native modules), the helperPath rewrite in src/unixTerminal.ts double-applies:

helperPath.replace('app.asar', 'app.asar.unpacked');

String.prototype.replace matches the first occurrence, which is the app.asar prefix inside app.asar.unpacked, producing app.asar.unpacked.unpacked/.../spawn-helper. That path does not exist, so on macOS every PTY spawn fails with posix_spawnp failed. (macOS is the only platform that spawns through the spawn-helper binary).

This PR extracts the resolution into resolveSpawnHelperPath() and skips each rewrite when the unpacked variant is already present. It is the same fix as #924 but with the logic factored into a pure, exported function so it can be regression-tested.

Changes

• src/unixTerminal.ts: extract resolveSpawnHelperPath(nativeDir, moduleDir); skip the app.asar / node_modules.asar rewrite when the .unpacked variant is already present.
• src/unixTerminal.test.ts: 4 regression tests covering packed → unpacked rewrite and already-unpacked (no double rewrite) for both app.asar and node_modules.asar.

Verification

• Pre-fix (regression tests fail): with the unguarded rewrite, 2 of 4 new tests fail with the exact bug path:
  '/app/node_modules.asar.unpacked.unpacked/node-pty/build/Release/spawn-helper' (expected .../app.asar.unpacked/...).
• Post-fix: 4/4 pass; full suite 28 passing on macOS; eslint src/ clean.
• End-to-end (real runtime): reproduced the failure inside the actual Electron runtime of a packaged app whose node-pty lives under app.asar.unpacked (posix_spawnp failed. for every spawn), then applied this change to that node-pty and confirmed a real PTY spawn of /bin/bash succeeds. Captured helper path before fix: .../app.asar.unpacked.unpacked/node_modules/node-pty/build/Release/spawn-helper; after: the correct single-unpacked path.

Notes

• Overlaps with fix(unix): don't rewrite app.asar.unpacked paths a second time #924 (same behavioral fix); the added value here is the pure-function extraction and regression tests, which fix(unix): don't rewrite app.asar.unpacked paths a second time #924 lacks. Happy to rebase or close in favor of fix(unix): don't rewrite app.asar.unpacked paths a second time #924 if it merges first.
• Related: fix(unix): validate spawn-helper exists before calling pty.fork() #917 (spawn-helper existence validation), macOS: spawn-helper shipped without execute bit in npm tarball causes posix_spawnp failures (pnpm) #850 (executable-bit packaging issue), Windows shells containing spaces aren't escaped correctly when using args in command line form (string) #91 in the downstream desktop app (posix_spawnp failed on macOS 26 in its minimal-mode persistent bash).

AI disclosure: this PR was developed with AI assistance (code + tests written with DeepSeek, reviewed and verified by a human) and is signed off by the human contributor.