Implement UpdatePolicy RPC

[whoAbhishekSah]

Context

The proto defines UpdatePolicy but the handler is unimplemented. This RPC allows clients to modify existing policies by ID.

Use case

CLI tools, direct API clients, and custom integrations that need fine-grained policy control. Not intended for SDK use — SDKs should use high-level role-based RPCs instead.

Implementation

• Accept policy ID and new role ID
• Delete the existing policy
• Create new policy with the updated role
• Execute both operations atomically (transaction)

Scope

• Implement UpdatePolicy handler
• Add authorization check (policymanage on resource)
• Validate role exists and is assignable to the resource type