fix(provider): tolerate trimmed image parts in litellm message conversion (#2475)

SessionManager clears image_base64 on past turns to save memory, and
exclude_none serialization drops the hollowed field entirely, so a
replayed history part can arrive as {'type': 'image_base64'} with no
payload. The converter accessed the missing key unconditionally and
raised KeyError on every turn after an image was sent.

Prefer the base64 payload when present, fall back to an image_url that
survived on the same element, and drop hollow parts otherwise (same
strategy as the existing file-part handling). Fixes #2469.

src/langbot/pkg/provider/modelmgr/requesters/litellmchat.py

@@ -747,9 +747,24 @@ class LiteLLMRequester(requester.ProviderAPIRequester):
                 converted_parts = []
                 for part in content:
                     if isinstance(part, dict) and part.get('type') == 'image_base64':
-                        part['image_url'] = {'url': part['image_base64']}
-                        part['type'] = 'image_url'
-                        del part['image_base64']
+                        # History trimming (SessionManager) clears image_base64
+                        # on past turns and exclude_none serialization drops
+                        # the key entirely, so the replayed part may carry no
+                        # payload. Prefer the base64 payload; fall back to an
+                        # image_url that survived on the same element; drop
+                        # hollow parts instead of raising KeyError (#2469).
+                        image_b64 = part.get('image_base64')
+                        fallback_url = None
+                        if not image_b64:
+                            raw_image_url = part.get('image_url')
+                            if isinstance(raw_image_url, dict):
+                                fallback_url = raw_image_url.get('url')
+                        if image_b64 or fallback_url:
+                            part['image_url'] = {'url': image_b64 or fallback_url}
+                            part['type'] = 'image_url'
+                            part.pop('image_base64', None)
+                        else:
+                            continue
                     # OpenAI-compatible chat models reject non-image file parts
                     # (audio/document base64 or url). These originate from Voice /
                     # File attachments — including ones replayed from conversation