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.

tests/providers/test_enforce_role_alternation.py

@@ -0,0 +1,128 @@
+"""Tests for LLMProvider._enforce_role_alternation."""
+
+from nanobot.providers.base import LLMProvider
+
+
+class TestEnforceRoleAlternation:
+    """Verify trailing-assistant removal and consecutive same-role merging."""
+
+    def test_empty_messages(self):
+        assert LLMProvider._enforce_role_alternation([]) == []
+
+    def test_no_change_needed(self):
+        msgs = [
+            {"role": "system", "content": "You are helpful."},
+            {"role": "user", "content": "Hi"},
+            {"role": "assistant", "content": "Hello!"},
+            {"role": "user", "content": "Bye"},
+        ]
+        result = LLMProvider._enforce_role_alternation(msgs)
+        assert len(result) == 4
+        assert result[-1]["role"] == "user"
+
+    def test_trailing_assistant_removed(self):
+        msgs = [
+            {"role": "user", "content": "Hi"},
+            {"role": "assistant", "content": "Hello!"},
+        ]
+        result = LLMProvider._enforce_role_alternation(msgs)
+        assert len(result) == 1
+        assert result[0]["role"] == "user"
+
+    def test_multiple_trailing_assistants_removed(self):
+        msgs = [
+            {"role": "user", "content": "Hi"},
+            {"role": "assistant", "content": "A"},
+            {"role": "assistant", "content": "B"},
+        ]
+        result = LLMProvider._enforce_role_alternation(msgs)
+        assert len(result) == 1
+        assert result[0]["role"] == "user"
+
+    def test_consecutive_user_messages_merged(self):
+        msgs = [
+            {"role": "user", "content": "Hello"},
+            {"role": "user", "content": "How are you?"},
+        ]
+        result = LLMProvider._enforce_role_alternation(msgs)
+        assert len(result) == 1
+        assert "Hello" in result[0]["content"]
+        assert "How are you?" in result[0]["content"]
+
+    def test_consecutive_assistant_messages_merged(self):
+        msgs = [
+            {"role": "user", "content": "Hi"},
+            {"role": "assistant", "content": "Hello!"},
+            {"role": "assistant", "content": "How can I help?"},
+            {"role": "user", "content": "Thanks"},
+        ]
+        result = LLMProvider._enforce_role_alternation(msgs)
+        assert len(result) == 3
+        assert "Hello!" in result[1]["content"]
+        assert "How can I help?" in result[1]["content"]
+
+    def test_system_messages_not_merged(self):
+        msgs = [
+            {"role": "system", "content": "System A"},
+            {"role": "system", "content": "System B"},
+            {"role": "user", "content": "Hi"},
+        ]
+        result = LLMProvider._enforce_role_alternation(msgs)
+        assert len(result) == 3
+        assert result[0]["content"] == "System A"
+        assert result[1]["content"] == "System B"
+
+    def test_tool_messages_not_merged(self):
+        msgs = [
+            {"role": "user", "content": "Hi"},
+            {"role": "assistant", "content": None, "tool_calls": [{"id": "1"}]},
+            {"role": "tool", "content": "result1", "tool_call_id": "1"},
+            {"role": "tool", "content": "result2", "tool_call_id": "2"},
+            {"role": "user", "content": "Next"},
+        ]
+        result = LLMProvider._enforce_role_alternation(msgs)
+        tool_msgs = [m for m in result if m["role"] == "tool"]
+        assert len(tool_msgs) == 2
+
+    def test_non_string_content_uses_latest(self):
+        msgs = [
+            {"role": "user", "content": [{"type": "text", "text": "A"}]},
+            {"role": "user", "content": "B"},
+        ]
+        result = LLMProvider._enforce_role_alternation(msgs)
+        assert len(result) == 1
+        assert result[0]["content"] == "B"
+
+    def test_original_messages_not_mutated(self):
+        msgs = [
+            {"role": "user", "content": "Hello"},
+            {"role": "user", "content": "World"},
+        ]
+        original_first = dict(msgs[0])
+        LLMProvider._enforce_role_alternation(msgs)
+        assert msgs[0] == original_first
+        assert len(msgs) == 2
+
+    def test_only_assistant_messages(self):
+        msgs = [
+            {"role": "assistant", "content": "A"},
+            {"role": "assistant", "content": "B"},
+        ]
+        result = LLMProvider._enforce_role_alternation(msgs)
+        assert result == []
+
+    def test_realistic_conversation(self):
+        msgs = [
+            {"role": "system", "content": "You are helpful."},
+            {"role": "user", "content": "What is 2+2?"},
+            {"role": "assistant", "content": "4"},
+            {"role": "user", "content": "And 3+3?"},
+            {"role": "user", "content": "(please be quick)"},
+            {"role": "assistant", "content": "6"},
+        ]
+        result = LLMProvider._enforce_role_alternation(msgs)
+        assert len(result) == 4
+        assert result[2]["role"] == "assistant"
+        assert result[3]["role"] == "user"
+        assert "And 3+3?" in result[3]["content"]
+        assert "(please be quick)" in result[3]["content"]