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/agents/checkpointer/provider.py

@@ -131,17 +131,19 @@ def get_checkpointer() -> Checkpointer:
     from deerflow.config.app_config import _app_config
     from deerflow.config.checkpointer_config import get_checkpointer_config
 
-    if _app_config is None:
-        # Only load config if it hasn't been initialized yet
-        # In tests, config may be set directly via set_checkpointer_config()
+    config = get_checkpointer_config()
+
+    if config is None and _app_config is None:
+        # Only load app config lazily when neither the app config nor an explicit
+        # checkpointer config has been initialized yet. This keeps tests that
+        # intentionally set the global checkpointer config isolated from any
+        # ambient config.yaml on disk.
         try:
             get_app_config()
         except FileNotFoundError:
-            # In test environments without config.yaml, this is expected
-            # Tests will set config directly via set_checkpointer_config()
+            # In test environments without config.yaml, this is expected.
             pass
-
-    config = get_checkpointer_config()
+        config = get_checkpointer_config()
     if config is None:
         from langgraph.checkpoint.memory import InMemorySaver