fix(context_compressor): treat streaming premature-close as transient error

Problem:
When a provider or proxy drops a streaming response mid-flight (httpcore
raises RemoteProtocolError: "incomplete chunked read", "peer closed
connection", "response ended prematurely", etc.), _generate_summary
would not classify it as a transient error.  Instead of retrying on the
main model, it entered the generic 60-second cooldown, leaving context
growing unbounded until the cooldown expired.  Issue #18458.

Root cause:
_is_connection_error in auxiliary_client.py did not match httpcore's
streaming premature-close error substrings.  context_compressor.py's
_generate_summary except block never called _is_connection_error, so
those errors fell through to the 60-second generic cooldown rather than
triggering the retry-on-main fallback path used for timeouts.

Fix:
1. auxiliary_client.py — extend _is_connection_error keyword list with:
   "incomplete chunked read", "peer closed connection",
   "response ended prematurely", "unexpected eof",
   "remoteprotocolerror", "localprotocolerror".
   Also guard the `from openai import ...` with try/except ImportError
   so the function works in environments without the openai package.
2. context_compressor.py — import _is_connection_error and call it in
   _generate_summary's except block as _is_streaming_closed.  Include
   _is_streaming_closed in the fallback-to-main condition (alongside
   _is_model_not_found, _is_timeout, _is_json_decode) and use the
   shorter 30s transient cooldown for streaming-closed errors.

Tests:
4 new regression tests in TestStreamingClosedFallback:
- test_incomplete_chunked_read_falls_back_to_main
- test_peer_closed_connection_falls_back_to_main
- test_streaming_closed_on_main_uses_short_cooldown  (stash-verified)
- test_non_streaming_unknown_error_still_uses_long_cooldown

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

agent/auxiliary_client.py

@@ -1840,10 +1840,12 @@ def _is_connection_error(exc: Exception) -> bool:
     distinct from API errors (4xx/5xx) which indicate the provider IS
     reachable but returned an error.
     """
-    from openai import APIConnectionError, APITimeoutError
-
-    if isinstance(exc, (APIConnectionError, APITimeoutError)):
-        return True
+    try:
+        from openai import APIConnectionError, APITimeoutError
+        if isinstance(exc, (APIConnectionError, APITimeoutError)):
+            return True
+    except ImportError:
+        pass
     # urllib3 / httpx / httpcore connection errors
     err_type = type(exc).__name__
     if any(kw in err_type for kw in ("Connection", "Timeout", "DNS", "SSL")):
@@ -1853,6 +1855,16 @@ def _is_connection_error(exc: Exception) -> bool:
         "connection refused", "name or service not known",
         "no route to host", "network is unreachable",
         "timed out", "connection reset",
+        # httpcore / httpx streaming premature-close errors.  These surface
+        # when a proxy or provider drops the connection mid-stream and are
+        # transient by nature — the request should be retried or rerouted.
+        # See issue #18458.
+        "incomplete chunked read",
+        "peer closed connection",
+        "response ended prematurely",
+        "unexpected eof",
+        "remoteprotocolerror",
+        "localprotocolerror",
     )):
         return True
     return False