fix(unix): don't rewrite app.asar.unpacked paths a second time

[arthur791004]

Summary

Fixes #923.

src/unixTerminal.ts#L19-L20 does:

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

String.prototype.replace with a string argument matches the first occurrence. When the resolved `helperPath` already contains `app.asar.unpacked/…` (e.g. a caller of node-pty that itself lives inside `app.asar.unpacked`, bypassing Electron's asar shim — see #923 for the concrete repro), the substring `app.asar` matches the prefix of `app.asar.unpacked` and we produce a bogus `…/app.asar.unpacked.unpacked/…` path. `posix_spawn` then fails with `ENOENT` and node-pty surfaces the misleading `posix_spawnp failed.`.

This PR guards each rewrite so it only fires when the unpacked variant isn't already present.

Test plan

• Existing unix-terminal tests still pass (`npm test` → 17/17).
• Manual repro on macOS arm64 from the linked issue's setup: before this patch `pty.spawn` throws `posix_spawnp failed.`; after this patch it spawns normally.

[fei (feiiiiii5)]

Hi — we hit this exact bug in a packaged Electron app on macOS 26 and verified your fix end-to-end. Sharing the evidence and a suggestion:

Reproduction (real runtime): loading node-pty 1.1.0 from an asar layout where it lives under app.asar.unpacked (the standard layout for native modules), every PTY spawn fails with posix_spawnp failed. even for /usr/bin/true. Captured helperPath passed to the native fork: .../app.asar.unpacked.unpacked/node_modules/node-pty/build/Release/spawn-helper. Plain child_process.spawn works, and a byte-level replication of the native spawn sequence with the correct helper path succeeds — confirming the failure is purely the rewritten helper path.

Your patch works: with the guarded rewrite applied, a real /bin/bash PTY spawn succeeds in the same runtime.

Suggestion — this needs tests: the inline rewrite is untestable as written. I've opened #953 which extracts resolveSpawnHelperPath() into a pure function and adds 4 regression tests (RED before the guard: the two "already-unpacked" cases produce app.asar.unpacked.unpacked; GREEN after; full macOS suite 28 passing, eslint clean). Happy to fold those tests into this PR instead if you'd prefer that shape, or rebase #953 on top of this one.