fix(providers): enforce role alternation for non-Claude providers

Some LLM providers (OpenAI-compat, Azure, vLLM, Ollama) reject requests
with consecutive same-role messages or trailing assistant messages. Add
_enforce_role_alternation() to merge consecutive same-role user/assistant
messages and strip trailing assistant messages before sending to the API.

nanobot/providers/azure_openai_provider.py

@@ -94,9 +94,11 @@ class AzureOpenAIProvider(LLMProvider):
     ) -> dict[str, Any]:
         """Prepare the request payload with Azure OpenAI 2024-10-21 compliance."""
         payload: dict[str, Any] = {
-            "messages": self._sanitize_request_messages(
-                self._sanitize_empty_content(messages),
-                _AZURE_MSG_KEYS,
+            "messages": self._enforce_role_alternation(
+                self._sanitize_request_messages(
+                    self._sanitize_empty_content(messages),
+                    _AZURE_MSG_KEYS,
+                )
             ),
             "max_completion_tokens": max(1, max_tokens),  # Azure API 2024-10-21 uses max_completion_tokens
         }

nanobot/providers/base.py

@@ -196,6 +196,42 @@ class LLMProvider(ABC):
         err = (content or "").lower()
         return any(marker in err for marker in cls._TRANSIENT_ERROR_MARKERS)
 
+    @staticmethod
+    def _enforce_role_alternation(messages: list[dict[str, Any]]) -> list[dict[str, Any]]:
+        """Merge consecutive same-role messages and drop trailing assistant messages.
+
+        Some providers (OpenAI-compat, Azure, vLLM, Ollama, etc.) reject requests
+        where the last message is 'assistant' (prefill not supported) or two
+        consecutive non-system messages share the same role.
+        """
+        if not messages:
+            return messages
+
+        merged: list[dict[str, Any]] = []
+        for msg in messages:
+            role = msg.get("role")
+            if (
+                merged
+                and role != "system"
+                and role not in ("tool",)
+                and merged[-1].get("role") == role
+                and role in ("user", "assistant")
+            ):
+                prev = merged[-1]
+                prev_content = prev.get("content") or ""
+                curr_content = msg.get("content") or ""
+                if isinstance(prev_content, str) and isinstance(curr_content, str):
+                    prev["content"] = (prev_content + "\n\n" + curr_content).strip()
+                else:
+                    merged[-1] = dict(msg)
+            else:
+                merged.append(dict(msg))
+
+        while merged and merged[-1].get("role") == "assistant":
+            merged.pop()
+
+        return merged
+
     @staticmethod
     def _strip_image_content(messages: list[dict[str, Any]]) -> list[dict[str, Any]] | None:
         """Replace image_url blocks with text placeholder. Returns None if no images found."""

nanobot/providers/openai_compat_provider.py

@@ -215,7 +215,7 @@ class OpenAICompatProvider(LLMProvider):
                 clean["tool_calls"] = normalized
             if "tool_call_id" in clean and clean["tool_call_id"]:
                 clean["tool_call_id"] = map_id(clean["tool_call_id"])
-        return sanitized
+        return self._enforce_role_alternation(sanitized)
 
     # ------------------------------------------------------------------
     # Build kwargs