Stop conversations crashing during compaction on tool-call turns (#1777)

context_compactor.maybe_compact built its summary text with
msg.get('content', '')[:2000], which raised
TypeError: 'NoneType' object is not subscriptable on assistant turns
whose content is None (turns that carried only native tool_calls).
Once a conversation crossed the 85% compaction threshold — reached
after only a few turns on small-context local models plus the large
agent prompt — every subsequent message failed ("send more than three
messages and it stops working").

Flatten message content to text first via a _content_as_text helper
(str passthrough, multimodal list blocks joined, None -> "") and
tolerate a missing role. Adds tests/test_context_compactor.py covering
the helper and a >=4-message conversation that forces compaction with
a None-content tool-call turn (fails before this change, passes after).

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/context_compactor.py

@@ -15,6 +15,26 @@ from core.models import ChatMessage
 
 logger = logging.getLogger(__name__)
 
+
+def _content_as_text(content: Any) -> str:
+    """Flatten a message's content to plain text.
+
+    Handles the three shapes that flow through history: a plain string, a
+    multimodal list of content blocks (vision/image attachments), and None
+    (assistant turns that carried only native tool_calls persist content as
+    None). Returns "" for anything without text so callers can safely slice
+    the result.
+    """
+    if isinstance(content, str):
+        return content
+    if isinstance(content, list):
+        return " ".join(
+            b.get("text", "") for b in content
+            if isinstance(b, dict) and b.get("text")
+        )
+    return ""
+
+
 COMPACT_THRESHOLD = 0.85  # Trigger compaction at 85% of context window
 SUMMARY_MAX_TOKENS = 1024
 SMALL_CONTEXT_LIMIT = 8192  # Models with context <= this get aggressive trimming
@@ -274,7 +294,7 @@ async def maybe_compact(
 
     # Build the text to summarize
     convo_text = "\n".join(
-        f"{msg['role'].upper()}: {msg.get('content', '')[:2000]}"
+        f"{msg.get('role', 'user').upper()}: {_content_as_text(msg.get('content'))[:2000]}"
         for msg in older
     )