fix(stockbit): tell a polling agent when Stockbit refuses the sign-in

A refused cookie in idx's browser was only a stderr warning, which nobody
reads under idx-mcp: stockbit_status showed `waiting` until the timeout, and
the timeout message did not mention the refusal.

- runtime::on_warning sends a thread's warnings to a sink as well as
  stderr, until its guard drops.
- The background sign-in puts its warnings in `sign_in.warning`, cleared
  once it signs in.
- The errors that end a browser wait (timeout, window closed, browser
  exited) name a refusal it hit.

Claude-Session: https://claude.ai/code/session_01RqPPVeVDrnLoP7NUZmLRxY

crates/idx-mcp/src/server.rs

@@ -189,7 +189,9 @@ IDX_STOCKBIT_REFRESH_TOKEN) and when its tokens expire; never the tokens. Withou
 get_orderbook fails and stockbit-provider quotes use Stockbit's delayed public page; the \
 user adds one with stockbit_login, or `idx stockbit login` in a terminal. After \
 stockbit_login, `sign_in.state` says whether that sign-in is waiting, signed_in or failed \
-(with `sign_in.error`).")]
+(with `sign_in.error`). While it waits, `sign_in.warning` is the latest problem it hit, such \
+as Stockbit refusing the session the user signed in with: pass it on, since the user may need \
+to sign in again.")]
     async fn stockbit_status(&self) -> ToolResult {
         blocking(login_status).await
     }