[Task]: make `ServerCallContext` class parameter mandatory

[sokoliva]

Currently ServerCallContext in an optional parameter in request handlers and task stores methods. Since we use ServerCallContext for mandatory authorization and authentication it should non-optional.

By making the ServerCallContext mandatory, we eliminate the risk of un-scoped database queries and ensure that the A2A server acts as a secure gatekeeper at the data layer.

[ishymko]

Fixed via #782 in 1.0-dev.