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/stockbit/parse.rs

@@ -124,8 +124,8 @@ pub fn status_error(status: u16, resolved: &str) -> IdxError {
     match status {
         403 => IdxError::UpstreamBlocked(
             "stockbit.com refused this client (HTTP 403 from its CDN); idx does not retry a \
-             block. Try again later or from another network; `--provider msn` serves quotes \
-             without Stockbit"
+             block. Try again later or from another network; the msn provider (`--provider msn`, \
+             or IDX_PROVIDER=msn for idx-mcp) serves quotes without Stockbit"
                 .to_string(),
         ),
         404 => IdxError::SymbolNotFound(resolved.to_string()),