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-mcp/src/server.rs

@@ -2,7 +2,9 @@
 //! `Idx` (as one CLI invocation would), so tools share the CLI's on-disk
 //! cache and ownership database.
 
+use std::cell::RefCell;
 use std::collections::BTreeMap;
+use std::rc::Rc;
 
 use idx_core::analysis::BandarQuery;
 use idx_core::analysis::bandar::{BandarReport, DEFAULT_TOP};
@@ -18,7 +20,7 @@ use idx_core::investor::{HolderSource, Investor};
 use idx_core::market::{ScreenFilter, ScreenQuery, ScreenRegion};
 use idx_core::movements::types::MovementQuery;
 use idx_core::ownership::types::ChangeRow;
-use idx_core::{CachePolicy, Fetched, Idx};
+use idx_core::{CachePolicy, Fetched, Idx, runtime};
 use rmcp::handler::server::router::tool::ToolRouter;
 use rmcp::handler::server::wrapper::Parameters;
 use rmcp::model::{CallToolResult, Implementation, ServerCapabilities, ServerConfig};
@@ -57,7 +59,9 @@ static CALL_SLOTS: Semaphore = Semaphore::const_new(4);
 
 const INSTRUCTIONS: &str = "Indonesian stock market (IDX) data. Market and financial \
 tools fetch live data (cached on disk) and wrap it as {data, source, fetched_at, stale}; \
-stale=true means an expired cache entry was served. Ownership tools read a local KSEI \
+stale=true means an expired cache entry was served. An answer (or error) may carry \
+`warnings`: what idx worked around, such as a refused Stockbit session that made history \
+fall back to yahoo, or the failure behind a stale entry; pass on any that need the user. Ownership tools read a local KSEI \
 database of holders above 1%: call ownership_status first to see which release dates \
 exist. Percentages ending in _bps are basis points (5494 = 54.94%). Errors carry a \
 stable code, e.g. SYMBOLNOTFOUND, RATELIMITED, INVALIDINPUT.";
@@ -745,13 +749,43 @@ where
         .acquire()
         .await
         .map_err(|err| ErrorData::internal_error(err.to_string(), None))?;
-    let result = tokio::task::spawn_blocking(move || f().and_then(to_json))
-        .await
-        .map_err(|err| ErrorData::internal_error(err.to_string(), None))?;
-    Ok(match result {
-        Ok(value) => CallToolResult::structured(value),
-        Err(err) => CallToolResult::structured_error(Value::Object(error_body(&err))),
+    let (result, warnings) = tokio::task::spawn_blocking(move || {
+        // idx-core warns on stderr, which the agent never sees; what this
+        // call warns about (a fallback, a stale cache, a partial answer)
+        // goes into its answer instead. The call runs on this thread.
+        let warnings = Rc::new(RefCell::new(Vec::<String>::new()));
+        let sink = {
+            let warnings = warnings.clone();
+            runtime::on_warning(move |message| {
+                let mut warnings = warnings.borrow_mut();
+                if !warnings.iter().any(|seen| seen == message) {
+                    warnings.push(message.to_string());
+                }
+            })
+        };
+        let result = f().and_then(to_json);
+        drop(sink);
+        (result, warnings.take())
     })
+    .await
+    .map_err(|err| ErrorData::internal_error(err.to_string(), None))?;
+    Ok(match result {
+        Ok(value) => CallToolResult::structured(with_warnings(value, warnings)),
+        Err(err) => CallToolResult::structured_error(with_warnings(
+            Value::Object(error_body(&err)),
+            warnings,
+        )),
+    })
+}
+
+/// Adds `warnings` to an answer when the call had any.
+fn with_warnings(mut value: Value, warnings: Vec<String>) -> Value {
+    if !warnings.is_empty()
+        && let Some(object) = value.as_object_mut()
+    {
+        object.insert("warnings".into(), json!(warnings));
+    }
+    value
 }
 
 /// Per-symbol failures as error bodies carrying their symbol.