Stop losing AI messages when a deploy interrupts them

[elias-ba]

Description

Oban's [:oban, :job, :stop] event had no handler attached, so a job that finished without raising was never seen by ObanManager. An AI chat message whose job was killed mid-run stayed :processing forever, and the panel stayed spinning.

Both events are now attached under their own handler ids. That separation matters: :telemetry detaches a handler from every event in an attach_many the first time it raises, so sharing one id meant a single bad :exception payload could silently take :stop down with it.

The Oban shutdown grace period also went up to 6 minutes, which is longer than the longest an AI job can now run. A deploy that lands mid-answer waits for the answer instead of severing it. There's a startup warning if the two ever drift back out of order.

Closes #__

Validation steps

1. Start the app and check both events have a handler:

   :telemetry.list_handlers([:oban, :job, :stop])
   :telemetry.list_handlers([:oban, :job, :exception])

   Each should include Lightning.ObanManager.handle_event/4.
2. mix test test/lightning/oban_manager_test.exs

AI Usage

• I have used Claude Code

Pre-submission checklist

• I have performed an AI review of my code
• I have updated the changelog. (for the whole stack, in #5074)
• I have ticked a box in "AI usage" in this PR

[github-actions]

Security Review ✅

• S0 (project scoping): N/A — telemetry handlers look up ChatMessage by a message_id from trusted Oban.Job.args set at enqueue time, not from a spoofable HTTP/LiveView param, and no new project-scoped query surface is introduced.
• S1 (authorization): N/A — no new controllers, LiveView events, or CRUD actions; changes are limited to telemetry attachment, Oban shutdown_grace_period, and internal Logger-level tweaks.
• S2 (audit trail): N/A — the only mutation is an internal ChatMessage status transition to :error for interrupted AI messages, which is not a project/instance configuration resource in the audit scope.

[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 90.6%. Comparing base (44aeb4a) to head (fe286cd).

Files with missing lines	Patch %	Lines
lib/lightning/ai_assistant/message_processor.ex	87.5%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #5069     +/-   ##
=======================================
- Coverage   90.6%   90.6%   -0.0%     
=======================================
  Files        421     421             
  Lines      20012   20014      +2     
=======================================
- Hits       18137   18134      -3     
- Misses      1875    1880      +5     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.