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/model_metadata.py

@@ -964,6 +964,10 @@ def parse_available_output_tokens_from_error(error_msg: str) -> Optional[int]:
     is_output_cap_error = (
         "max_tokens" in error_lower
         and ("available_tokens" in error_lower or "available tokens" in error_lower)
+    ) or (
+        # OpenRouter/Nous phrasing of the same condition.
+        "in the output" in error_lower
+        and "maximum context length" in error_lower
     )
     if not is_output_cap_error:
         return None
@@ -982,6 +986,19 @@ def parse_available_output_tokens_from_error(error_msg: str) -> Optional[int]:
             tokens = int(match.group(1))
             if tokens >= 1:
                 return tokens
+
+    # OpenRouter/Nous format: "maximum context length is N … (A of text input,
+    # B of tool input, C in the output)". Available output = ctx - text - tool.
+    _m_ctx = re.search(r'maximum context length is (\d+)', error_lower)
+    _m_parts = re.search(
+        r'\((\d+)\s+of text input,\s*(\d+)\s+of tool input,\s*(\d+)\s+in the output\)',
+        error_lower,
+    )
+    if _m_ctx and _m_parts:
+        _available = int(_m_ctx.group(1)) - int(_m_parts.group(1)) - int(_m_parts.group(2))
+        if _available >= 1:
+            return _available
+
     return None