Fix duel crash due to failing lookups (#37606)

[winnerspiros]

Summary by CodeRabbit

• Bug Fixes
  • Improved duel notification handling to more reliably process user requests and prevent potential edge case issues.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 9e12f0b4-a979-43e9-ad8e-bf2d46f86e72

📥 Commits

Reviewing files that changed from the base of the PR and between 86898d3 and 3924fc7.

📒 Files selected for processing (1)

• osu.Game/Screens/OnlinePlay/Matchmaking/Queue/QueueController.cs

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📝 Walkthrough

Walkthrough

The duel notification dispatch in QueueController is refactored from a continuation-based async pattern to an async local method. The new flow adds explicit null safety and uses the Scheduler for posting notifications, while removing an unused using directive.

Changes

Duel Notification Dispatch

Layer / File(s)	Summary
Async Pattern Refactoring osu.Game/Screens/OnlinePlay/Matchmaking/Queue/QueueController.cs	onMatchmakingDuelIssued replaces ContinueWith continuation with async local handleDuelRequestAsync method. Flow now awaits users.GetUserAsync(duel.UserId), checks for null, and schedules DuelNotification via Scheduler.Add instead of forcing non-null dereference.
Import Cleanup osu.Game/Screens/OnlinePlay/Matchmaking/Queue/QueueController.cs	Removed unused using osu.Framework.Extensions; while retaining using osu.Framework.Extensions.ObjectExtensions;.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Poem

A rabbit hops through async trails,
Where continuations once did sail,
Now local methods smooth the way,
With null-checks bright as morning's ray— 🐰✨
Notifications queued to play!

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'Fix duel crash due to failing lookups' directly relates to the main change: handling null user lookups in duel notification logic to prevent crashes.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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