fix: ensure OAuth server cleanup on error before callback resolves

[gnapse]

Summary

• Expose cleanup function from startCallbackServer so callers can stop the server on error
• Wrap OAuth flow in try-catch and call cleanup when open() or subsequent steps throw before the callback promise settles
• Prevents server from running until 3-minute timeout when errors occur early in the flow

Test plan

• npm run type-check passes
• npm test passes (551 tests)
• Added new test: 'calls cleanup when open() throws'

🤖 Generated with Claude Code

[henningmu]

Looks good, thanks 👍