fix(streaming): stop socket read timeout from preempting stale-stream detector (#43570)

* fix(streaming): stop socket read timeout from preempting stale-stream detector

The stale-stream detector is deliberately scaled to 180-300s so reasoning
models (e.g. Opus) can pause mid-stream during extended thinking. But the
httpx socket read timeout stayed at a flat 120s for cloud providers and fired
first, tearing down healthy reasoning streams before the detector (which owns
retry + diagnostics) could act. Symptom: every Copilot/Opus turn dies with
ReadTimeout at a consistent ~125s and never completes.

Floor the cloud socket read timeout at the stale-stream timeout so it can no
longer fire before the detector. Local providers and explicit
HERMES_STREAM_READ_TIMEOUT / request_timeout_seconds overrides are unchanged.

* test(streaming): pin read-timeout >= stale-stream invariant for cloud reasoning streams

Cover the contract that the httpx socket read timeout is never shorter than
the stale-stream detector for cloud providers on the default: small contexts
floor to 180s, >=50K to 240s, >=100K to 300s; explicit overrides win; local
providers and the unresolved-value fallback are unaffected.

agent/chat_completion_helpers.py

@@ -1698,6 +1698,14 @@ def interruptible_streaming_api_call(agent, api_kwargs: dict, *, on_first_delta=
     # poll loop uses this to detect stale connections that keep receiving
     # SSE keep-alive pings but no actual data.
     last_chunk_time = {"t": time.time()}
+    # Stale-stream patience, shared between the httpx socket read timeout
+    # (built in ``_call_chat_completions`` below) and the stale-stream detector
+    # (computed further down, before the worker thread starts).  Initialized
+    # here so the read-timeout builder can floor itself at the stale value and
+    # never fire before the detector.  ``None`` until the detector value is
+    # resolved, so the builder degrades to its plain default if it ever runs
+    # first.
+    _stream_stale_timeout = None
 
     def _fire_first_delta():
         if not first_delta_fired["done"] and on_first_delta:
@@ -1734,6 +1742,26 @@ def interruptible_streaming_api_call(agent, api_kwargs: dict, *, on_first_delta=
                     "Local provider detected (%s) — stream read timeout raised to %.0fs",
                     agent.base_url, _stream_read_timeout,
                 )
+            elif (
+                _stream_read_timeout == 120.0
+                and _stream_stale_timeout is not None
+                and _stream_stale_timeout != float("inf")
+                and _stream_stale_timeout > _stream_read_timeout
+            ):
+                # Cloud reasoning models (e.g. Opus) routinely pause mid-stream
+                # for minutes during extended thinking.  The stale-stream
+                # detector is deliberately scaled up to tolerate this (180–300s,
+                # see the stale-timeout block below), but the raw httpx socket
+                # read timeout defaulted to a flat 120s and fired *first* —
+                # tearing down a healthy reasoning stream before the stale
+                # detector (which owns retry + diagnostics) could act.  Keep the
+                # socket read timeout in step with the detector so it no longer
+                # preempts it.
+                _stream_read_timeout = _stream_stale_timeout
+                logger.debug(
+                    "Cloud reasoning stream — read timeout raised to %.0fs to "
+                    "match stale-stream detector", _stream_read_timeout,
+                )
         # Cap connect/pool at 60s even when provider timeout is higher.
         # connect/pool cover TCP handshake, not model inference.
         _conn_cap = min(_base_timeout, 60.0) if _provider_timeout_cfg is not None else 30.0