Wrap socket/timeout errors in HTTP client as HttpClientError

stream_chat_completion (and the LLM stream/chat paths) only caught HTTPError, so a
request exceeding request_timeout_s raised a raw TimeoutError mid-stream that escaped
_run_one_request (which only catches HttpClientError), propagated through the probe,
and crashed the whole trial ("failed: timed out"). A timed-out request is a failed
request (SLO miss), not a trial crash. Catch OSError (covers TimeoutError, URLError,
ConnectionError) after HTTPError and wrap it. Exposed by lowering request_timeout_s
to 180s on the 27B run.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/aituner/http_client.py

@@ -179,6 +179,9 @@ def chat_completion(
     except urllib.error.HTTPError as exc:
         detail = exc.read().decode("utf-8", errors="replace")
         raise HttpClientError(f"llm_completion failed: {exc.code} {detail}") from exc
+    except OSError as exc:
+        # TimeoutError (socket.timeout), URLError, ConnectionError all subclass OSError.
+        raise HttpClientError(f"llm_completion failed: {exc}") from exc
 
 
 def stream_text_completion(
@@ -232,6 +235,8 @@ def stream_text_completion(
     except urllib.error.HTTPError as exc:
         detail = exc.read().decode("utf-8", errors="replace")
         raise HttpClientError(f"stream_text_completion failed: {exc.code} {detail}") from exc
+    except OSError as exc:
+        raise HttpClientError(f"stream_text_completion failed: {exc}") from exc
     return "".join(parts)
 
 
@@ -293,6 +298,10 @@ def stream_chat_completion(
     except urllib.error.HTTPError as exc:
         detail = exc.read().decode("utf-8", errors="replace")
         raise HttpClientError(f"stream_chat_completion failed: {exc.code} {detail}") from exc
+    except OSError as exc:
+        # A request that exceeds request_timeout_s raises TimeoutError mid-stream;
+        # treat it as a failed request (SLO miss), not a crashed trial.
+        raise HttpClientError(f"stream_chat_completion failed: {exc}") from exc
     ttft_ms = None if first_token_at is None else (first_token_at - start) * 1000.0
     if completion_tokens is None and chunk_token_count > 0:
         completion_tokens = chunk_token_count