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/tests/test_checkpointer.py

@@ -5,6 +5,7 @@ from unittest.mock import MagicMock, patch
 
 import pytest
 
+import deerflow.config.app_config as app_config_module
 from deerflow.agents.checkpointer import get_checkpointer, reset_checkpointer
 from deerflow.config.checkpointer_config import (
     CheckpointerConfig,
@@ -17,9 +18,11 @@ from deerflow.config.checkpointer_config import (
 @pytest.fixture(autouse=True)
 def reset_state():
     """Reset singleton state before each test."""
+    app_config_module._app_config = None
     set_checkpointer_config(None)
     reset_checkpointer()
     yield
+    app_config_module._app_config = None
     set_checkpointer_config(None)
     reset_checkpointer()
 
@@ -75,7 +78,8 @@ class TestGetCheckpointer:
         """get_checkpointer should return InMemorySaver when not configured."""
         from langgraph.checkpoint.memory import InMemorySaver
 
-        cp = get_checkpointer()
+        with patch("deerflow.agents.checkpointer.provider.get_app_config", side_effect=FileNotFoundError):
+            cp = get_checkpointer()
         assert cp is not None
         assert isinstance(cp, InMemorySaver)