fix: always strip old functionCall/functionResponse from LS history

The function call stripping was only happening when no custom tools
were present. But even with custom tools injected, the LS history
contains functionCall/functionResponse parts for LS-internal tools
that we stripped, causing MALFORMED_FUNCTION_CALL. Now always strip
regardless of custom tools presence.

src/mitm/modify.rs

@@ -206,8 +206,13 @@ pub fn modify_request(body: &[u8], tool_ctx: Option<&ToolContext>) -> Option<Vec
                 changes.push("remove toolConfig (no tools)".to_string());
             }
         }
+    }
 
-        // Also strip functionCall/functionResponse from conversation history
+    // ── 3b. ALWAYS strip old functionCall/functionResponse from history ───
+    // Even when custom tools are injected, the LS history contains function
+    // call parts for LS-internal tools we stripped. Google rejects these as
+    // MALFORMED_FUNCTION_CALL because the referenced tools don't exist.
+    if STRIP_ALL_TOOLS {
         if let Some(contents) = json
             .pointer_mut("/request/contents")
             .and_then(|v| v.as_array_mut())