Ignore non-string email thread bodies (#1654)

tests/test_email_thread_parser_nonstring.py

@@ -0,0 +1,13 @@
+from src.email_thread_parser import parse_thread
+
+
+def test_parse_thread_ignores_non_string_bodies():
+    assert parse_thread(123, {"bad": True}) is None
+    assert parse_thread(["<blockquote>bad</blockquote>"], None) is None
+
+
+def test_parse_thread_still_handles_plaintext_quotes():
+    turns = parse_thread(None, "hi\n\nOn Tue, Alice wrote:\n> older")
+
+    assert turns
+    assert turns[0]["level"] == 0