Two fixes from live debugging:

1. broker_execute_stream: remove the "the model is thinking..." keepalive chunk.
   It leaked into the assistant reply (saved turns began with the placeholder) and
   isn't wanted — we just wait for the real response. The engine waits ~5min for a
   load and the broker sends protocol-level `pending` keepalives, so liveness holds
   without injecting content. Reverts the streaming body to the simple
   retry-on-connection/transient-5xx form (no queue/keepalive).

2. _clamp_max_tokens: always resolve a concrete max_tokens, never None. The main
   stream/generate paths pass request.max_tokens bare; when the client omits it the
   GGUF backend's `max_tokens or 512` default truncated replies at ~512 tokens
   mid-sentence. Now: model-level cap is authority (client honored only if smaller);
   no cap configured -> keep client value, else default 2048.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RdMufYvtTbtGDWsiZVoXce