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

docs/CODE_CONTEXT_RELIABILITY_PLAN.md

@@ -272,3 +272,19 @@ Updated: 2026-04-16 01:41 (KST)
 - Remaining follow-up:
   - extend pre-request tool-trace validation so the flattening/orphan repair count trends toward zero rather than being logged after repair
   - replace more mojibake prompt/status strings in active Code execution paths with English equivalents
+
+Updated: 2026-04-16 01:57 (KST)
+
+- Delivered in this pass:
+  - Phase 4 partial delivery:
+    - `AgentMessageInvariantHelper.cs` now normalizes historical tool traces before the request leaves the agent loop.
+    - structured assistant tool-call messages without matching `tool_result` now flatten into plain assistant transcript text.
+    - orphan `tool_result` messages now flatten into plain user transcript text instead of relying only on late OpenAI payload repair.
+    - `AgentLoopLlmRequestPreparationService.cs` clones the query window first, then applies normalization, so request cleanup does not mutate stored conversation history.
+    - `AgentLoopContextReliability.cs` now logs tool-trace repair counts inside the `query_context` transition for run-by-run observability.
+  - Phase 5 partial delivery:
+    - `SessionLearningCollector.cs` was rewritten with English-only comments and English injection text.
+    - `AgentLoopDiagnosticsFormatter.cs` no longer emits mojibake-prone compaction status text in active Code paths.
+- Remaining follow-up:
+  - measure whether `tool_trace_repair` counts keep trending down in long Code runs after this preflight normalization
+  - continue replacing older mojibake strings outside the active Code execution path