Fix session export 500 on multimodal/None message content (#1984)

txt/html/md export joined and string-munged message.content directly, so a
multimodal turn (content is a list of blocks) crashed export with a TypeError
on join (txt) / AttributeError on .replace (html), and None content (tool-only
assistant turns) rendered as the literal 'None'. Add a _content_to_text helper
that flattens string/list/None to plain text and apply it at the three export
sites. JSON export is unchanged (it serializes structured content correctly).
Plain-string content is returned unchanged, so existing exports are identical.

Co-authored-by: ghreprimand <203024559+ghreprimand@users.noreply.github.com>

tests/test_session_export_nonstring_content.py

@@ -0,0 +1,50 @@
+"""Regression: session export must tolerate non-string message content.
+
+A message's ``content`` is a plain string for normal turns, but a multimodal
+list of content blocks for image/vision turns, and ``None`` for assistant turns
+that persisted only native tool_calls. The txt/html/md exporters in
+``routes/session_routes.py`` joined and string-munged ``content`` directly, so:
+
+  - txt:  ``"\n".join([..., <list>, ...])``      -> TypeError
+  - html: ``<list>.replace("&", "&amp;")``        -> AttributeError
+  - md:   ``f"{<list>}"``                          -> raw Python repr in output
+
+``_content_to_text`` coerces all three shapes to plain text so export degrades
+gracefully instead of returning a 500.
+"""
+from routes.session_routes import _content_to_text
+
+
+def test_plain_string_passes_through_unchanged():
+    assert _content_to_text("hello world") == "hello world"
+    assert _content_to_text("") == ""
+
+
+def test_multimodal_list_flattens_to_its_text_blocks():
+    content = [
+        {"type": "text", "text": "describe this"},
+        {"type": "image_url", "image_url": {"url": "data:image/png;base64,AAAA"}},
+        {"type": "text", "text": "thanks"},
+    ]
+    assert _content_to_text(content) == "describe this\nthanks"
+
+
+def test_none_content_becomes_empty_string():
+    # Assistant turns carrying only native tool_calls persist content as None.
+    assert _content_to_text(None) == ""
+
+
+def test_list_without_text_blocks_is_empty_not_crash():
+    assert _content_to_text([{"type": "image_url", "image_url": {"url": "x"}}]) == ""
+    assert _content_to_text([]) == ""
+
+
+def test_coerced_output_survives_the_export_operations():
+    # The exact operations that previously crashed must now succeed.
+    history = ["plain", [{"type": "text", "text": "img turn"}], None]
+    texts = [_content_to_text(c) for c in history]
+    # txt export path
+    assert "\n".join(texts) == "plain\nimg turn\n"
+    # html export path
+    for t in texts:
+        assert isinstance(t.replace("&", "&amp;"), str)