fix: multi-round tool history rewrite and finishReason handling

- Add ToolRound struct to pair function calls with results per-round
- Replace single-match history rewrite (broke after first round) with
  multi-round loop that rewrites ALL placeholder model turns
- Fix tool result name fallback: use positional index instead of always
  picking the first call
- Set is_complete for any finishReason (FUNCTION_CALL, MAX_TOKENS, etc.)
  not just STOP — prevents response_complete flag from never being set
- Legacy fallback: responses.rs path (single-round via last_calls +
  pending_results) still works when tool_rounds is empty
- Add tests: multi-round rewrite, single-round legacy, no-op, and
  FUNCTION_CALL/MAX_TOKENS finishReason handling

src/mitm/store.rs

@@ -60,6 +60,18 @@ pub struct PendingToolResult {
     pub result: serde_json::Value,
 }
 
+/// A single round of tool calling: the model's function calls paired with
+/// the client's execution results.
+///
+/// In multi-step tool use, each round has its own calls and results.
+/// This preserves per-turn data so history rewriting can map each
+/// "Tool call completed" model turn to the correct functionCall/functionResponse.
+#[derive(Debug, Clone)]
+pub struct ToolRound {
+    pub calls: Vec<CapturedFunctionCall>,
+    pub results: Vec<PendingToolResult>,
+}
+
 /// An upstream error captured from Google's API response.
 /// Stored by the MITM proxy so API handlers can return it to the client
 /// instead of hanging forever waiting for a response that won't come.
@@ -152,6 +164,9 @@ pub struct MitmStore {
     call_id_to_name: Arc<RwLock<HashMap<String, String>>>,
     /// Last captured function calls (for conversation history rewriting).
     last_function_calls: Arc<RwLock<Vec<CapturedFunctionCall>>>,
+    /// Multi-round tool call history for correct per-turn history rewriting.
+    /// Set by completions/responses handler, consumed by modify_request.
+    tool_rounds: Arc<RwLock<Vec<ToolRound>>>,
 
     // ── Cascade correlation ──────────────────────────────────────────────
     /// Active cascade ID set by the API layer before sending a message.
@@ -223,6 +238,7 @@ impl MitmStore {
             pending_tool_results: Arc::new(RwLock::new(Vec::new())),
             call_id_to_name: Arc::new(RwLock::new(HashMap::new())),
             last_function_calls: Arc::new(RwLock::new(Vec::new())),
+            tool_rounds: Arc::new(RwLock::new(Vec::new())),
             active_cascade_id: Arc::new(RwLock::new(None)),
             captured_response_text: Arc::new(RwLock::new(None)),
             captured_thinking_text: Arc::new(RwLock::new(None)),
@@ -511,6 +527,16 @@ impl MitmStore {
         self.last_function_calls.read().await.clone()
     }
 
+    /// Store multi-round tool call history for correct per-turn history rewriting.
+    pub async fn set_tool_rounds(&self, rounds: Vec<ToolRound>) {
+        *self.tool_rounds.write().await = rounds;
+    }
+
+    /// Take (consume) multi-round tool call history.
+    pub async fn take_tool_rounds(&self) -> Vec<ToolRound> {
+        std::mem::take(&mut *self.tool_rounds.write().await)
+    }
+
     // ── Direct response capture (bypass LS) ──────────────────────────────
 
     /// Set (replace) the captured response text.