fix: replace async-exit-hook with exit-hook to preserve process.exitCode

[AmineMaila]

Description

Replaces async-exit-hook with exit-hook to fix a bug where process.exitCode was always overridden to 0 on process exit.

async-exit-hook unconditionally calls process.exit(0) after its cleanup hooks run, ignoring any process.exitCode set by the application. The exit-hook package (by the same author, sindresorhus) hooks into process exit events without forcing the exit code, preserving whatever process.exitCode was set.

I encountered this while using vitest and embedded-postgres together for testing — when a test failed, vitest sets process.exitCode = 1, but async-exit-hook would force it back to 0, causing CI pipelines to incorrectly report the test run as successful.

Changes

• packages/embedded-postgres/package.json: Replaced async-exit-hook dependency with exit-hook
• packages/embedded-postgres/src/index.ts: Updated import from AsyncExitHook to asyncExitHook, removed the done callback parameter from gracefulShutdown, added a 5-second wait timeout
• packages/embedded-postgres/tests/index.test.ts: Added test verifying that process.exitCode is preserved when using EmbeddedPostgres
• packages/embedded-postgres/tests/helpers/exit-code-test.mjs: Helper script for the exit code test

Testing

• All 8 existing + new tests pass
• Verified that a child process using EmbeddedPostgres with process.exitCode = 42 exits with code 42 (not 0)