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>
