[feature] Add automatic session orchestration service

[marwannettour]

Parent epic: #300

Summary

Add client-side orchestration for creating an automatic trusted session.

Scope

• Validate target ClientId values before calling the backend.
• Validate that each target is locally trusted by the creator.
• Create the cloud session through the existing creation flow.
• Start automatic invitations after session creation.
• Track per-target progress and terminal results.

Acceptance criteria

• A client service orchestrates automatic trusted session creation.
• Local validation rejects empty, duplicate, self-target, too-many-targets, and untrusted targets.
• The existing cloud session creation flow is reused.
• The creator remains the first session member.
• Per-target statuses are available for UI and CLI consumers.
• Unit tests cover validation and orchestration success/failure paths.