fix: retry httpx timeout family as transient LLM errors

Live incident: the compatible-mode provider stalled mid-stream after
200 OK (~17s without data) and raised httpx.ReadTimeout, which was not
in the retriable exception-name set, so the call failed after 1
attempt and killed the whole run. Add ReadTimeout/ConnectTimeout/
WriteTimeout/PoolTimeout/TimeoutError/ConnectError to the transient
set in LLMErrorHandlingMiddleware (retry with backoff, then a
transient fallback message instead of generic), and add ReadTimeout/
ConnectTimeout to ai_writing's classify_exception for model
switching.

.gitignore

@@ -130,6 +130,8 @@ offline-backend-20260512/backend/tests/*
 # Writing-mode first-report pipeline enforcement regressions.
 !offline-backend-20260512/backend/tests/test_writing_mode_report_middleware.py
 !offline-backend-20260512/backend/tests/test_writing_rewrite_prompt.py
+# LLM call timeout retry classification regression coverage.
+!offline-backend-20260512/backend/tests/test_llm_error_handling_middleware.py
 deploy
 .history