fix(copilot): skip transcript upload when builder does not cover full session prefix

When transcript download fails or returns invalid content on a session with
prior messages, the transcript builder only contains the current turn. Uploading
it with message_count=len(session.messages) would overstate coverage, causing
_build_query_message to skip gap reconstruction on the next turn.

Add _transcript_covers_prefix flag that is set to False when download fails,
and check it before uploading to prevent partial transcripts from being stored
as if they covered the entire session history.

autogpt_platform/backend/backend/copilot/sdk/service.py

@@ -864,6 +864,12 @@ async def stream_chat_completion_sdk(
     resume_file: str | None = None
     transcript_builder = TranscriptBuilder()
     sdk_cwd = ""
+    # True when transcript_builder represents a full prefix of session history.
+    # First turn (<=1 prior message) is fully covered even without a download.
+    # Set to True when load_previous succeeds; stays False when download fails
+    # on a session with prior messages, preventing a partial upload that would
+    # mislead _build_query_message into skipping gap reconstruction next turn.
+    _transcript_covers_prefix = True
 
     # Acquire stream lock to prevent concurrent streams to the same session
     lock = AsyncClusterLock(
@@ -1002,12 +1008,14 @@ async def stream_chat_completion_sdk(
                     )
             else:
                 logger.warning("%s Transcript downloaded but invalid", log_prefix)
+                _transcript_covers_prefix = False
         elif config.claude_agent_use_resume and user_id and len(session.messages) > 1:
             logger.warning(
                 "%s No transcript available (%d messages in session)",
                 log_prefix,
                 len(session.messages),
             )
+            _transcript_covers_prefix = False
 
         yield StreamStart(messageId=message_id, sessionId=session_id)
 
@@ -1719,6 +1727,14 @@ async def stream_chat_completion_sdk(
                         log_prefix,
                         entry_count,
                     )
+                elif not _transcript_covers_prefix:
+                    logger.warning(
+                        "%s Skipping transcript upload — builder does not "
+                        "cover full session prefix (entries=%d, session=%d)",
+                        log_prefix,
+                        entry_count,
+                        len(session.messages),
+                    )
                 else:
                     logger.info(
                         "%s Uploading transcript (entries=%d, bytes=%d)",