fix: use process.execPath for .js module spawning on all platforms @W-21984392@

[WillieRuemmele]

Summary

• Remove the win32 platform guard in spawn.ts so .js module paths are invoked via Node on all platforms, not just Windows
• Use process.execPath instead of the string 'node' — ensures the bundled Node binary is used in standalone tarball deployments
• Add unit tests covering .js path handling (Unix + args passthrough + paths with spaces) and non-.js path passthrough

Fixes #1293

Work Item

@W-21984392@: [oclif][ISSUE] spawn fails with ENOENT on standalone tarballs (UNIX)

Bug Reproduction

The bug occurs when a .js file (e.g., npm-cli.js) has a shebang pointing to a Node interpreter that doesn't exist on the host (common in standalone tarball deployments where only the bundled node exists):

=== OLD behavior: spawn .js directly ===
Spawning: /tmp/repro/npm-cli.js

ERROR: ENOENT — spawn /tmp/repro/npm-cli.js ENOENT
^ The OS tries the shebang interpreter (#!/usr/bin/nonexistent-node)
  which does not exist → ENOENT

=== NEW behavior: spawn via process.execPath ===
Spawning: /path/to/node /tmp/repro/npm-cli.js

Exit code: 0
Output: npm install success

✓ Fix works — process.execPath bypasses the broken shebang

On Unix, ENOENT from spawn() can mean "missing shebang interpreter," not just "target file missing" — which makes this error confusing to diagnose.

Proof of Work

• Tests: 32 passing (0 failing)
• Lint: clean
• Type check: clean
• Build: clean

Test plan

• Reproduce ENOENT with bogus shebang .js file spawned directly
• Verify fix: same .js file spawned via process.execPath succeeds
• Unit test: .js paths invoked via process.execPath on Unix
• Unit test: .js paths with spaces in the path
• Unit test: args passed correctly after the module path
• Unit test: non-.js paths remain unchanged
• Integration: plugins:install on standalone tarball (Linux, no system Node.js)
• Integration: plugins:install on Windows (paths with spaces)
• Integration: plugins:install with npm-installed CLI (system node)

[WillieRuemmele]

Reproduction & Verification

Ran the actual spawn() function from src/spawn.ts against a .js file with a bogus shebang (#!/usr/bin/nonexistent-node) to simulate a standalone tarball environment where the shebang interpreter doesn't exist:

Before fix (old code with win32 guard):

Spawning .js file via our spawn() function: /tmp/repro/npm-cli.js
✗ FAILED — spawn /tmp/repro/npm-cli.js ENOENT

After fix (using process.execPath on all platforms):

Spawning .js file via our spawn() function: /tmp/repro/npm-cli.js
✓ SUCCESS — output: npm install success

The fix correctly routes .js module paths through the running Node binary (process.execPath) instead of trying to exec them directly (which relies on the shebang interpreter existing).