[feature] Define automatic session DTOs and failure reasons

[marwannettour]

Parent epic: #300

Summary

Define the shared DTOs, statuses, and failure reasons required by automatic trusted sessions.

Scope

• Add request/response contracts for automatic trusted session creation.
• Add target-level result models.
• Add explicit failure reasons for UI, CLI, logs, and tests.
• Keep the contracts independent from the UI and CLI implementations.

Acceptance criteria

• Shared contracts exist in ByteSync.Common.
• The request targets stable ClientId values.
• The response exposes per-target statuses.
• Failure reasons cover validation, trust, grant, challenge, join, and timeout failures.
• Contracts are covered by serialization or mapping tests where relevant.