[feature] Resolve trusted targets for automatic sessions

[marwannettour]

Parent epic: #300

Summary

Add target resolution for automatic trusted sessions, mapping stable ClientId values to connected ClientInstanceId values.

Scope

• Resolve each requested target ClientId to an online client instance.
• Reject offline targets.
• Reject ambiguous targets when several instances are online for the same ClientId in the MVP.
• Enforce the session member limit server-side.
• Return structured failure reasons.

Acceptance criteria

• A server-side service or handler resolves automatic session targets.
• Offline targets return a deterministic failure.
• Ambiguous online instances return a deterministic failure.
• The creator cannot target its own stable ClientId.
• The current session member limit is enforced.
• Unit tests cover success, offline, ambiguous, self-target, and too-many-members cases.