fix: remove misleading model.max_tokens suggestion from thinking-exhausted error (#11626)

The 'Thinking Budget Exhausted' user-facing error message advised users to
'set model.max_tokens in config.yaml'. That config key is documented but
intentionally not wired through to the API call in CLI/gateway paths — we
omit max_tokens by default so the inference server uses its full output
budget (llama-server -1=infinity, vLLM max_model_len-prompt_len, etc.).

Users followed the suggestion, saw no change, and kept filing bugs (see
closed #4404, #10917, #6955 and PRs #5001/#6080/#6446/#6707/#7075/#8804/
#10924/#11173/#11268 — all reporting the same misdirection).

Replace the misleading suggestion with an actionable one: switch models
via /model. Lowering reasoning effort remains the primary remediation.

run_agent.py

@@ -9303,8 +9303,7 @@ class AIAgent:
                                 "and had none left for the actual response.\n\n"
                                 "To fix this:\n"
                                 "→ Lower reasoning effort: `/thinkon low` or `/thinkon minimal`\n"
-                                "→ Increase the output token limit: "
-                                "set `model.max_tokens` in config.yaml"
+                                "→ Or switch to a larger/non-reasoning model with `/model`"
                             )
                             self._cleanup_task_resources(effective_task_id)
                             self._persist_session(messages, conversation_history)