fix(npm): propagate CLI exit code from npm wrapper (related to #214)

[stantheman0128]

Summary

Related to #214. The quoting half of that issue was already fixed in merged #215 (execFileSync with argv array). This PR addresses the residual exit-code propagation when the npm wrapper shells out to the platform binary.

execFileSync throws on non-zero CLI exit, so npx zeabur service exec (and similar) printed a Node stack trace and always exited 1, discarding deliberate exit codes from commands like internal/cmd/service/exec/exec.go and internal/cmd/server/exec/exec.go.

Switch to spawnSync + process.exitCode so the wrapper forwards the binary's status (including signal-terminated runs).

AI disclosure

AI-assisted patch; human-reviewed by Stan Shih (stantheman0128) before publish.

Verification / Evidence

Windows 11, fake binary harness (same argv-preservation cases as #214 discussion):

# Before (origin/main npm/index.js with execFileSync):
#   exit 3 -> wrapper exits 1 + Node stack trace
#   exit 137 -> wrapper exits 1 + Node stack trace

# After (spawnSync + process.exitCode):
#   exit 3 -> 3
#   exit 137 -> 137
#   5/5 argv cases pass (nested quotes, backslashes, empty arg, shell metachars literal)
#   ENOENT -> single-line error, exit 1 (no stack)

go test ./...   # green

Published npm 0.21.0 tarball on registry also carries the #215 argv fix; this PR is only the exit-code half.

What was not tested

• Live npx zeabur against a remote Zeabur API (local fake-binary harness only).

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