Fix _parse_msg_content corrupting JSON-array-like text messages on reload (#2060)

_parse_msg_content deserializes stored multimodal content (image/audio
blocks) back into a list. It treated ANY string starting with '[{' and
containing the substring "type" as serialized content, requiring only
that each element be a dict — never that "type" be a real content-block
kind. So a plain text message whose content happens to be a JSON array
of typed objects (e.g. a user pasting an API schema sample like
[{"type": "object", ...}]) was silently parsed from str into a list on
the next hydration, destroying the original string. This runs on every
session load from the DB (_db_to_session -> get_session). Restrict the
round-trip to non-empty lists whose every element is a dict whose
"type" is a recognized block kind (text/image/image_url/audio/...);
real multimodal content (verified: document_processor emits exactly
these) still round-trips, JSON-looking text is left untouched.

tests/test_parse_msg_content_jsonlike_string.py

@@ -0,0 +1,80 @@
+"""A plain text message that merely *looks* like a JSON array of objects must
+NOT be silently re-parsed into a list on reload.
+
+_parse_msg_content de-serializes multimodal (image/audio) content back into a
+list of content blocks. The old heuristic accepted ANY string that started
+with "[{" and contained the substring '"type"'. A user who pasted an API
+schema / sample such as `[{"type": "object", "name": "foo"}]` therefore had
+their text message permanently corrupted into a Python list on the next
+session hydration. The fix restricts the round-trip to lists whose elements
+are all recognized content-block types (text/image_url/audio/...).
+"""
+import tempfile
+import uuid
+
+import pytest
+from sqlalchemy import create_engine
+from sqlalchemy.orm import sessionmaker
+from sqlalchemy.pool import NullPool
+
+import core.database as cdb
+from core.database import Session as DbSession
+from core.models import ChatMessage
+
+_TMPDB = tempfile.NamedTemporaryFile(suffix=".db", delete=False)
+_ENGINE = create_engine(
+    f"sqlite:///{_TMPDB.name}",
+    connect_args={"check_same_thread": False},
+    poolclass=NullPool,
+)
+cdb.Base.metadata.create_all(_ENGINE)
+_TS = sessionmaker(bind=_ENGINE, autoflush=False, autocommit=False)
+
+
+@pytest.fixture
+def manager(monkeypatch):
+    import core.session_manager as sm
+    monkeypatch.setattr(sm, "SessionLocal", _TS)
+    mgr = sm.SessionManager.__new__(sm.SessionManager)
+    mgr.sessions = {}
+    return mgr
+
+
+def _make_session(sid, owner="alice"):
+    db = _TS()
+    try:
+        db.add(DbSession(id=sid, owner=owner, name="chat",
+                         endpoint_url="http://x", model="gpt-4o",
+                         archived=False, message_count=1))
+        db.commit()
+    finally:
+        db.close()
+
+
+def test_jsonlike_user_string_not_corrupted(manager):
+    sid = "sess-" + uuid.uuid4().hex[:8]
+    _make_session(sid)
+    text = '[{"type": "object", "name": "foo"}]'
+    msgs = [ChatMessage(role="user", content=text)]
+    assert manager.replace_messages(sid, msgs) is True
+
+    manager.sessions.clear()
+    reloaded = manager.get_session(sid)
+    # Must come back as the ORIGINAL STRING, not silently parsed into a list.
+    assert isinstance(reloaded.history[0].content, str)
+    assert reloaded.history[0].content == text
+
+
+def test_real_multimodal_content_still_round_trips(manager):
+    sid = "sess-" + uuid.uuid4().hex[:8]
+    _make_session(sid)
+    multimodal = [
+        {"type": "text", "text": "what is this?"},
+        {"type": "image_url", "image_url": {"url": "data:image/png;base64,AAAA"}},
+    ]
+    msgs = [ChatMessage(role="user", content=multimodal)]
+    assert manager.replace_messages(sid, msgs) is True
+
+    manager.sessions.clear()
+    reloaded = manager.get_session(sid)
+    assert reloaded.history[0].content == multimodal