[dev-v5][Menu] Update FluentMenu not opening

[MarvinKlein1508]

Pull Request

📖 Description

microsoft/fluentui#36018 changed the behaviour how we need to register the trigger slot in FluentMenu. This PR addresses this change.

✅ Checklist

General

• I have added tests for my changes.
• I have tested my changes.
• I have updated the project documentation to reflect my changes.
• I have read the CONTRIBUTING documentation and followed the standards for this project.

Component-specific

• I have added a new component
• I have added Unit Tests for my new component
• I have modified an existing component
• I have validated the Unit Tests for an existing component

⏭ Next Steps

[Copilot]

Pull request overview

This PR updates the FluentMenu JavaScript/TypeScript initialization to align with the Fluent UI web-components behavior change (per fluentui/fluentui#36018), ensuring the menu trigger slot registration is performed in the way the updated web component expects.

Changes:

• Replaced manual mutation of menu.slottedTriggers with a call to slottedTriggersChanged(...) to register the trigger.
• Removed the call to menu.setComponent() as part of initialization.