test: Unit tests for Outbox Inspector endpoints

[samtrion]

User Story

As a developer maintaining Pulse.AspNetCore, I want unit tests for all Outbox Inspector endpoints, so that I can detect regressions.

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Requirements

• Tests.Unit/AspNetCore/OutboxInspectorEndpointsTests.cs:
  • All seven endpoints with mocked IOutboxManagement.
  • 404 when message or dead-letter not found.
  • Custom BasePath applied correctly.

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Acceptance Criteria

• All tests pass.

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Dependencies

• Requires feat: Outbox Inspector Minimal API endpoints (MapOutboxInspector)