[test] Add automatic session contract tests

[marwannettour]

Parent epic: #300

Summary

Add contract and unit tests for automatic trusted session validation and failure mapping.

Scope

• Cover DTO defaults and serialization where relevant.
• Cover client-side validation.
• Cover server-side target resolution.
• Cover failure reason mapping for UI and CLI consumers.

Acceptance criteria

• Tests use FluentAssertions.
• Tests do not use NUnit.Framework.Legacy or ClassicAssert.
• Validation failures map to explicit automatic session failure reasons.
• Server target resolution failures map to explicit automatic session failure reasons.
• Tests follow Arrange-Act-Assert.