Migration-planner: agent api ignores jwt source_id claim

A flaw was found in migration-planner. The agent-API middleware processes JSON Web Tokens (JWTs) for authentication, but its UpdateSourceInventory and UpdateAgentStatus handlers fail to validate the source_id claim within these tokens against the requested source ID. This oversight allows an authenticated attacker with a valid agent token to manipulate data across different tenants, leading to a complete collapse of tenant isolation. This could result in unauthorized overwriting of victim inventory, planting of malicious credential URLs, or corruption of migration assessments.