Support threadless MCP connections without event channels (#34408)

## What changed

- Make the MCP connection manager's event sender optional.
- Skip startup notifications when a caller has no session event stream.
- Decline interactive MCP elicitations in that mode while allowing the
  underlying operation to continue.
- Update threadless resource reads, status snapshots, and connector discovery
  to use the channel-free path.

## Testing

- Extend the app-server MCP resource test to verify that a threadless resource
  read declines an elicitation and still returns the resource contents.

GitOrigin-RevId: 65fea152adbc02a78e09a40cae4134b6dccffd34

codex-rs/app-server/Cargo.toml

@@ -26,7 +26,6 @@ workspace = true
 
 [dependencies]
 anyhow = { workspace = true }
-async-channel = { workspace = true }
 base64 = { workspace = true }
 axum = { workspace = true, default-features = false, features = [
     "http1",