[test] Add automatic trusted session integration tests

[marwannettour]

Parent epic: #300

Summary

Add integration test scenarios for automatic trusted sessions.

Scope

• Cover the happy path with already trusted clients.
• Cover offline target failure.
• Cover missing trust failure.
• Cover missing grant failure after issue [feature] Epic: Add trusted-device grants for automatic joins #295 is implemented.
• Cover invalid challenge failure after issue [feature] Epic: Add signed challenge flow for automatic joins #298 is implemented.
• Cover timeout while waiting for selected targets.

Acceptance criteria

• At least one integration test validates the automatic trusted session happy path.
• Integration tests validate the main rejection paths that can be tested without UI.
• Grant-dependent tests are linked to [feature] Epic: Add trusted-device grants for automatic joins #295.
• Challenge-dependent tests are linked to [feature] Epic: Add signed challenge flow for automatic joins #298.
• Tests run separately from build, following repository guidelines.