fix: prevent tool_rounds cross-cascade contamination causing hangs

Root cause: proxy.rs eagerly pushed tool rounds via push_tool_round_calls
when intercepting Google's functionCall response. These stale rounds leaked
into LS follow-up requests, producing malformed history that Google timed
out on (60s 'no upstream response').

Changes:
- Remove push_tool_round_calls from proxy.rs response interception
- proxy.rs: use get_tool_rounds (non-destructive) instead of take_tool_rounds
  so accumulated rounds persist across multiple LS requests per cascade
- responses.rs/gemini.rs: build rounds via take+push+set pattern — each
  handler accumulates its own rounds from get_last_function_calls + results
- completions.rs: unchanged (set_tool_rounds replaces from messages)
- clear_tools: also clears tool_rounds to prevent stale data between sessions
- store.rs: add get_tool_rounds (non-destructive clone) method

src/api/responses.rs

@@ -269,14 +269,19 @@ pub(crate) async fn handle_responses(
                 result: result_value,
             });
         }
-        // Attach results to the latest open ToolRound (pushed by proxy.rs)
-        state
-            .mitm_store
-            .attach_tool_round_results(pending)
-            .await;
+        // Build a ToolRound from the MITM-captured function calls + client results.
+        // get_last_function_calls() has the calls from Google's previous response.
+        // We take existing accumulated rounds and append this new round.
+        let last_calls = state.mitm_store.get_last_function_calls().await;
+        let mut rounds = state.mitm_store.take_tool_rounds().await;
+        rounds.push(crate::mitm::store::ToolRound {
+            calls: last_calls,
+            results: pending,
+        });
+        state.mitm_store.set_tool_rounds(rounds).await;
         info!(
             count = tool_results.len(),
-            "Stored tool results for MITM injection (attached to tool round)"
+            "Stored tool results for MITM injection (built tool round)"
         );
     }