feat(app): let an extension supply the A2A handler

[jjamroga]

Problem

grpcserver registers the A2A service only when A2AHandler is non-nil, and core/pkg/app never sets it. A controller built on app.Start can create AgentInstances but cannot talk to them:

Unimplemented: unknown service lf.a2a.v1.A2AService

core/cmd/controller-v2 does set it, but that is package main, so the capability is unreachable to anything embedding app.Start.

Change

ExtensionConfig gains an A2AHandler field, passed through to grpcserver. That's it — one file, two lines of code.

Nothing else changes. The field defaults to nil and grpcserver skips registration exactly as it does today, so this is inert unless an extension opts in.

It also needs no reordering: getExtensionConfig is called with the DbClient in BootstrapConfig well before the gRPC server is constructed, so an extension can build a handler with a2agateway.New and return it.

Alternative considered

I first went at this by exporting core/internal/grpcserver and adding a public constructor for the store, so the v2 stack could be assembled outside package main (#2543). This is much smaller, keeps the internals internal, and covers the actual need — so I've closed that one in favour of this.

The larger export still has an independent argument: it would let an external module stand up the gRPC surface with a real Postgres and fakes for a2agateway's dialer and agentinstance's actor client, and test AgentInstance lifecycle, ownership checks, A2A routing and DefaultMethodPolicies without a cluster or Substrate. Happy to revive it if that's wanted, but it shouldn't be bundled with this.

Testing

go build ./..., go vet, and go test ./core/pkg/app/ ./core/internal/grpcserver/ all pass.