5.1 Connect invitation acceptance to the existing join-session flow

[marwannettour]

Parent epic: #311

Summary

Create coordination that joins a session after a trusted-session invitation is accepted.

Requirements

• Create a service such as ITrustedSessionJoiner.
• Accept a validated invitation payload.
• Call the existing join-session flow with session ID and session password.
• Return a result with session ID, success flag, and optional error.

Acceptance criteria

• Accepted invitations can trigger the existing join flow.
• Join failures are surfaced clearly.
• The flow reuses existing session-join behavior.