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/tests/golden/tools.json

@@ -604,7 +604,7 @@
     "name": "stockbit_login"
   },
   {
-    "description": "Whether a Stockbit session is stored (or seeded by IDX_STOCKBIT_REFRESH_TOKEN) and when its tokens expire; never the tokens. Without one, 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`).",
+    "description": "Whether a Stockbit session is stored (or seeded by IDX_STOCKBIT_REFRESH_TOKEN) and when its tokens expire; never the tokens. Without one, 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`). 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.",
     "inputSchema": {
       "properties": {},
       "type": "object"