fix(openrouter): add x-grok-conv-id header for Grok models to improve prompt cache hit rates (carve-out of #22708)

Pass session_id through to provider profile build_api_kwargs_extras so
the OpenRouter profile can attach an xAI cache-affinity header
(x-grok-conv-id: <session-id>) for x-ai/grok-* models. xAI prompt
cache requires server affinity via this header — without it the cache
is poisoned and Grok prompt-cache hit rates drop dramatically on
multi-turn sessions.

Carve-out of #22708 by Ninso112. The original PR bundled a /diff
slash command, a zsh completion fix (already on main via #22802),
and holographic memory null-guards. This salvage keeps just the
Grok header work — small, targeted, and well-tested. Other
contributors and changes preserved for separate review.

Closes #22705.

agent/transports/chat_completions.py

@@ -448,6 +448,7 @@ class ChatCompletionsTransport(ProviderTransport):
                 qwen_session_metadata=params.get("qwen_session_metadata"),
                 model=model,
                 ollama_num_ctx=params.get("ollama_num_ctx"),
+                session_id=params.get("session_id"),
             )
         )
         api_kwargs.update(top_level_from_profile)