test: Unit tests for Command Dead Letter Inspector endpoints

[samtrion]

User Story

As a developer maintaining Pulse.AspNetCore, I want unit tests for the Command Dead Letter Inspector.

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Requirements

• Tests.Unit/AspNetCore/CommandDeadLetterInspectorEndpointsTests.cs:
  • All five endpoints with mocked ICommandDeadLetterManagement.
  • 404 when entry not found.

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Acceptance Criteria

• All tests pass.

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Dependencies

• Requires feat: Command Dead Letter Inspector Minimal API endpoints (MapCommandDeadLetterInspector)