fix(stream+output-cap): guard empty streams and parse OpenRouter output-cap errors (#40589)

Two isolated reliability fixes:
- chat_completion_helpers: raise on a zero-chunk stream (no finish_reason,
  no content/reasoning/tool_calls) so retry handles it instead of
  fabricating a successful empty turn.
- model_metadata: parse the OpenRouter/Nous output-cap error phrasing
  ("maximum context length is N ... (A of text input, B of tool input,
  C in the output)") so parse_available_output_tokens_from_error returns
  a real cap and the caller stops looping on it.

Salvaged from #40405 (@ashishpatel26) — took the two stream/error-parsing
fixes. The PR also bundled compression-state changes (on_session_start
clearing _previous_summary; cron session-id prefix preservation, #38788);
those touch the compression hot path and are split out for separate review.

Co-authored-by: ashishpatel26 <ashishpatel26@users.noreply.github.com>

agent/chat_completion_helpers.py

@@ -1936,6 +1936,20 @@ def interruptible_streaming_api_call(agent, api_kwargs: dict, *, on_first_delta=
                     ),
                 ))
 
+        # Zero-chunk guard: stream yielded nothing usable — a provider/upstream
+        # error or malformed SSE, not a legitimate empty completion. Raise so the
+        # retry machinery handles it instead of fabricating a successful turn.
+        if (
+            finish_reason is None
+            and not content_parts
+            and not reasoning_parts
+            and not tool_calls_acc
+        ):
+            raise RuntimeError(
+                "Provider returned an empty stream with no finish_reason "
+                "(possible upstream error or malformed SSE response)."
+            )
+
         effective_finish_reason = finish_reason or "stop"
         if has_truncated_tool_args:
             effective_finish_reason = "length"