feat(grpc): add tool approval flow for Chat/SessionChat RPCs

[cidrblock]

Summary

The REST API supports an interactive tool approval flow for chat:

1. POST /api/chat streams SSE events including approval_request when a tool invocation needs human approval
2. POST /api/chat/:chatId/approve resolves the pending approval (approve or deny)

The gRPC Chat and SessionChat RPCs support streaming responses but have no equivalent approval mechanism. This means gRPC clients cannot replicate the dashboard's interactive chat-with-approval UX.

Proposal

Design a gRPC-native approval flow. Options include:

1. Bidirectional streaming — extend Chat/SessionChat to accept client-to-server approval messages mid-stream
2. Separate approval RPC — add an ApproveToolCall(chatId, toolCallId, approved) unary RPC that resolves pending approvals, mirroring the REST endpoint
3. Callback pattern — use the existing VSCodeStream bidirectional channel to relay approval requests/responses

Option 2 is simplest and most consistent with the REST pattern.

Context

Identified during gRPC config parity audit in #32.