refactor(agent): tighten comments, extract constant, strengthen edge case test

- Extract synthetic user message string to module-level constant
- Tighten comments in _snip_history recovery branch
- Strengthen no-user edge case test to verify safety net interaction

nanobot/agent/runner.py

@@ -933,14 +933,15 @@ class AgentRunner:
                     kept = kept[i:]
                     break
             else:
-                # No user message in the kept window — walk backwards through
-                # non_system to find the nearest user message and keep it plus
-                # everything after it.  Providers like GLM reject requests
-                # where the first non-system message is not ``user`` (error 1214).
+                # Recover nearest user message from outside the kept window;
+                # GLM rejects system→assistant (error 1214).  Budget is
+                # intentionally exceeded — oversized beats invalid.
                 for idx in range(len(non_system) - 1, -1, -1):
                     if non_system[idx].get("role") == "user":
                         kept = non_system[idx:]
                         break
+                # If no user exists at all, _enforce_role_alternation
+                # will insert a synthetic one as a safety net.
             start = find_legal_message_start(kept)
             if start:
                 kept = kept[start:]

nanobot/providers/base.py

@@ -77,6 +77,9 @@ class GenerationSettings:
     reasoning_effort: str | None = None
 
 
+_SYNTHETIC_USER_CONTENT = "(conversation continued)"
+
+
 class LLMProvider(ABC):
     """Base class for LLM providers."""
 
@@ -417,7 +420,7 @@ class LLMProvider(ABC):
         for i, msg in enumerate(merged):
             if msg.get("role") != "system":
                 if msg.get("role") == "assistant" and not msg.get("tool_calls"):
-                    merged.insert(i, {"role": "user", "content": "(conversation continued)"})
+                    merged.insert(i, {"role": "user", "content": _SYNTHETIC_USER_CONTENT})
                 break
 
         return merged

tests/agent/test_runner.py

@@ -2909,3 +2909,12 @@ def test_snip_history_no_user_at_all_falls_back_gracefully(monkeypatch):
     assert isinstance(trimmed, list)
     # Must have at least system.
     assert any(m.get("role") == "system" for m in trimmed)
+    # The _enforce_role_alternation safety net must be able to fix whatever
+    # _snip_history returns here — verify it produces a valid sequence.
+    from nanobot.providers.base import LLMProvider
+    fixed = LLMProvider._enforce_role_alternation(trimmed)
+    non_system = [m for m in fixed if m["role"] != "system"]
+    if non_system:
+        assert non_system[0]["role"] in ("user", "tool"), (
+            f"Safety net should ensure first non-system is user/tool, got {non_system[0]['role']}"
+        )

tests/providers/test_enforce_role_alternation.py

@@ -1,6 +1,6 @@
 """Tests for LLMProvider._enforce_role_alternation."""
 
-from nanobot.providers.base import LLMProvider
+from nanobot.providers.base import LLMProvider, _SYNTHETIC_USER_CONTENT
 
 
 class TestEnforceRoleAlternation:
@@ -208,7 +208,7 @@ class TestEnforceRoleAlternation:
         result = LLMProvider._enforce_role_alternation(msgs)
         non_system = [m for m in result if m["role"] != "system"]
         assert non_system[0]["role"] == "user"
-        assert non_system[0]["content"] == "(conversation continued)"
+        assert non_system[0]["content"] == _SYNTHETIC_USER_CONTENT
         # The original assistant should follow.
         assert non_system[1]["role"] == "assistant"