fix: normalize structured LLM content in serialization and memory updater (#1215)

* fix: normalize ToolMessage structured content in serialization

When models return ToolMessage content as a list of content blocks
(e.g. [{"type": "text", "text": "..."}]), the UI previously displayed
the raw Python repr string instead of the extracted text.

Replace str(msg.content) with the existing _extract_text() helper in
both _serialize_message() and stream() to properly normalize
list-of-blocks content to plain text.

Fixes #1149

Also fixes the same root cause as #1188 (characters displayed one per
line when tool response content is returned as structured blocks).

Added 11 regression tests covering string, list-of-blocks, mixed,
empty, and fallback content types.

* fix(memory): extract text from structured LLM responses in memory updater

When LLMs return response content as list of content blocks
(e.g. [{"type": "text", "text": "..."}]) instead of plain strings,
str() produces Python repr which breaks JSON parsing in the memory
updater. This caused memory updates to silently fail.

Changes:
- Add _extract_text() helper in updater.py for safe content normalization
- Use _extract_text() instead of str(response.content) in update_memory()
- Fix format_conversation_for_update() to handle plain strings in list content
- Fix subagent executor fallback path to extract text from list content
- Replace print() with structured logging (logger.info/warning/error)
- Add 13 regression tests covering _extract_text, format_conversation,
  and update_memory with structured LLM responses

* fix: address Copilot review - defensive text extraction + logger.exception

- client.py _extract_text: use block.get('text') + isinstance check (prevent KeyError/TypeError)
- prompt.py format_conversation_for_update: same defensive check for dict text blocks
- executor.py: type-safe text extraction in both code paths, fallback to placeholder instead of str(raw_content)
- updater.py: use logger.exception() instead of logger.error() for traceback preservation

* Apply suggestions from code review

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* fix: preserve chunked structured content without spurious newlines

* fix: restore backend unit test compatibility

---------

Co-authored-by: Exploreunive <Exploreunive@users.noreply.github.com>
Co-authored-by: Willem Jiang <willem.jiang@gmail.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

backend/packages/harness/deerflow/subagents/executor.py

@@ -288,13 +288,23 @@ class SubagentExecutor:
                     if isinstance(content, str):
                         result.result = content
                     elif isinstance(content, list):
-                        # Extract text from list of content blocks for final result only
+                        # Extract text from list of content blocks for final result only.
+                        # Concatenate raw string chunks directly, but preserve separation
+                        # between full text blocks for readability.
                         text_parts = []
+                        pending_str_parts = []
                         for block in content:
                             if isinstance(block, str):
-                                text_parts.append(block)
-                            elif isinstance(block, dict) and "text" in block:
-                                text_parts.append(block["text"])
+                                pending_str_parts.append(block)
+                            elif isinstance(block, dict):
+                                if pending_str_parts:
+                                    text_parts.append("".join(pending_str_parts))
+                                    pending_str_parts.clear()
+                                text_val = block.get("text")
+                                if isinstance(text_val, str):
+                                    text_parts.append(text_val)
+                        if pending_str_parts:
+                            text_parts.append("".join(pending_str_parts))
                         result.result = "\n".join(text_parts) if text_parts else "No text content in response"
                     else:
                         result.result = str(content)
@@ -302,7 +312,27 @@ class SubagentExecutor:
                     # Fallback: use the last message if no AIMessage found
                     last_message = messages[-1]
                     logger.warning(f"[trace={self.trace_id}] Subagent {self.config.name} no AIMessage found, using last message: {type(last_message)}")
-                    result.result = str(last_message.content) if hasattr(last_message, "content") else str(last_message)
+                    raw_content = last_message.content if hasattr(last_message, "content") else str(last_message)
+                    if isinstance(raw_content, str):
+                        result.result = raw_content
+                    elif isinstance(raw_content, list):
+                        parts = []
+                        pending_str_parts = []
+                        for block in raw_content:
+                            if isinstance(block, str):
+                                pending_str_parts.append(block)
+                            elif isinstance(block, dict):
+                                if pending_str_parts:
+                                    parts.append("".join(pending_str_parts))
+                                    pending_str_parts.clear()
+                                text_val = block.get("text")
+                                if isinstance(text_val, str):
+                                    parts.append(text_val)
+                        if pending_str_parts:
+                            parts.append("".join(pending_str_parts))
+                        result.result = "\n".join(parts) if parts else "No text content in response"
+                    else:
+                        result.result = str(raw_content)
                 else:
                     logger.warning(f"[trace={self.trace_id}] Subagent {self.config.name} no messages in final state")
                     result.result = "No response generated"