코드탭 tool trace 사전 정규화와 인코딩 정리 적용

- AgentMessageInvariantHelper에 전송 직전 structured tool trace 정규화 로직을 추가해 missing tool_result assistant와 orphan tool_result를 plain transcript로 평탄화함

- AgentLoopLlmRequestPreparationService에서 query view를 clone한 뒤 normalization을 적용하고 query_context 로그에 tool_trace_repair 메타를 남기도록 확장함

- SessionLearningCollector와 AgentLoopDiagnosticsFormatter의 깨진 문자열과 주석을 영어 기준으로 정리해 active Code 경로의 mojibake 노출을 줄임

- AgentMessageInvariantHelperTests, AgentLoopLlmRequestPreparationServiceTests를 보강하고 dotnet build 및 targeted dotnet test(34 통과, 경고/오류 0)로 검증함

src/AxCopilot.Tests/Services/AgentLoopLlmRequestPreparationServiceTests.cs

@@ -86,4 +86,34 @@ public class AgentLoopLlmRequestPreparationServiceTests
         result.SendMessages[1].MetaKind.Should().Be("code_working_set");
         result.SendMessages.Last().Content.Should().Contain("[TOOL_REQUIRED]");
     }
+
+    [Fact]
+    public void Prepare_ShouldNormalizeBrokenHistoricalToolTraceBeforeSend()
+    {
+        var queryMessages = new List<ChatMessage>
+        {
+            new()
+            {
+                Role = "assistant",
+                Content = """{"_tool_use_blocks":[{"type":"text","text":"Inspecting source"},{"type":"tool_use","id":"call-missing","name":"file_read","input":{"path":"App.xaml"}}]}"""
+            },
+            new()
+            {
+                Role = "user",
+                Content = "fix the latest build failure"
+            }
+        };
+
+        var result = AgentLoopLlmRequestPreparationService.Prepare(
+            queryMessages,
+            totalToolCalls: 1,
+            forceInitialToolCallEnabled: true,
+            injectPreCallToolReminder: true,
+            noToolCallLoopRetry: 0);
+
+        result.FlattenedStructuredAssistantCount.Should().Be(1);
+        result.ConvertedOrphanToolResultCount.Should().Be(0);
+        result.SendMessages[0].Content.Should().Be("Inspecting source\n[previous tool call] file_read");
+        queryMessages[0].Content.Should().StartWith("{\"_tool_use_blocks\"");
+    }
 }