fix(mcp): hand a call's warnings to the agent

idx-core reports what it works around (history falling back to yahoo when
Stockbit refuses the session, a stale cache served after a failed fetch, a
movements query cut short) as warnings on stderr. An agent never reads
idx-mcp's stderr, so it saw `source: "yahoo"` or `stale: true` with no reason.

- Each tool call collects its warnings with runtime::on_warning and returns
  them as `warnings` on the answer or the error, when there are any.
- The stale-cache warning names the failure behind it.
- The mock's session API 403 is the real session API's block message; the
  public page's block names IDX_PROVIDER=msn as well as `--provider msn`.

Goldens: error-intraday-blocked (json, table) carry the reworded block hint.

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

crates/idx-core/src/api/mock.rs

@@ -187,6 +187,10 @@ impl MockStockbitProvider {
                 "Stockbit rejected the session; run `idx stockbit login` again".to_string(),
             ));
         }
+        // The session API's own block, as `exodus::fetch` reports it.
+        if std::env::var("IDX_MOCK_STOCKBIT_STATUS").as_deref() == Ok("403") {
+            return Err(stockbit::exodus::blocked());
+        }
         if let Some(err) = self.failure(symbol) {
             return Err(err);
         }