fix(cli): treat tab-close after successful create as completion

[kaiweijw]

Summary

The Mode A wizard's heartbeat watchdog detects a closed browser tab ~3.6s after it disappears and emits WizardOutcome::Cancelled. But if the user clicked Create service right before closing — backend already committed an active service — the resulting "✗ Wizard cancelled. No service was created." message is misleading and prompts users to re-run, creating duplicates.

This PR captures successful AiKey create responses inside the existing proxy sniff and treats them as the implicit completion ack when the soft-failure outcomes (heartbeat watchdog, max-duration timeout) fire.

closes #601

What changed

• New KeysResponseSignal enum + pure classify_keys_response helper extracted from the inline sniff at handle_proxy. Same lifecycle behavior for pending_auth placeholders; new ActiveCreated signal captures successful creates.
• New ServerState.completed_ai_key: Mutex<Option<Value>> snapshots the response body when the active-create signal fires AND the flow is FlowKind::AiKey. Display-once flows (api-key-create, rotate, node-register) still require the explicit /api/proxy/complete ack so secrets don't get accidentally surfaced.
• Watchdog and timeout branches in the outcome select prefer the captured success and emit WizardOutcome::AiKeyCompleted(value); the printed message is updated to reflect what happened.
• Ctrl+C and explicit cancel button preserved as hard cancel — those are unambiguous user gestures.

Test plan

• cargo test -p nyxid-cli classify_keys_response (7 new classifier tests)
• cargo test -p nyxid-cli (288 unit + freshness)
• Manual: NYXID_WIZARD_NO_OPEN=1 nyxid service add llm-openai --label "QA tab close test", open wizard URL, paste API key, click Create service, immediately close the tab. Confirm CLI prints "Browser stopped responding (tab closed?) — using last successful create as completion." and shows the wizard summary instead of "No service was created".
• Regression: explicit Cancel button still terminates with WizardOutcome::Cancelled.
• Regression: Ctrl+C during a successful create still terminates with WizardOutcome::Cancelled (the user's gesture is honored).