Increase shared code-mode host capacity safely (#59590)

## Why

Shared hosts serve independent chats and their subagents. Larger callback workloads need room for unrelated work and control messages, while gRPC completion bursts can exhaust HTTP/2's small-DATA-frame budget and disconnect shared sessions.

## What changed

- Raise the active-cell limit from 128 to 2,048 and the pending delegate callback limit from 1,024 to 4,096. Expand request and transport queues to leave room for completion bursts and control traffic.
- Negotiate larger callback budgets through the stdio `delegate-capacity-4096` capability and gRPC `max_pending_delegate_calls` field. Preserve the 1,024-callback budget for legacy clients and enforce both session and shared host limits on gRPC.
- Limit concurrent gRPC completion RPCs to 64 across a provider's sessions, allowing queued completions to be canceled without waiting for delivery capacity.

## Testing

Add stdio and gRPC integration tests covering 256 cells and 4,096 callbacks across shared sessions while unrelated work completes. Add tests for callback budget isolation and reuse, rejection of zero capacity, and cancellation of queued completions. Include an ignored resource-intensive test for full host capacity.

GitOrigin-RevId: e85bfa24cc46cc8a735b464d80adf8066a6a6d3a

codex-rs/code-mode-protocol/src/grpc/codex.code_mode.v1.proto

@@ -60,6 +60,10 @@ message OpenSessionRequest {
   optional SessionCellExecutionLimits cell_execution_limits = 1;
   // Absent preserves the legacy client-side truncation behavior.
   optional OutputTruncation output_truncation = 2;
+  // Maximum pending tool callbacks the client can accept.
+  // Absent preserves the legacy 1,024 callback budget. Hosts may impose a lower
+  // shared limit, and older hosts ignore this field and retain their own limit.
+  optional uint32 max_pending_delegate_calls = 3;
 }
 
 enum OutputTruncation {