fix: preserve Telegram usernames with underscores

tests/test_text_sanitizer.py

@@ -0,0 +1,24 @@
+import sys
+from pathlib import Path
+
+sys.path.insert(0, str(Path(__file__).resolve().parents[1] / "backend"))
+
+from bot.utils.text_sanitizer import sanitize_display_name, sanitize_username, username_for_display
+
+
+def test_sanitize_username_preserves_underscore_suffixes():
+    assert sanitize_username("ik_end") == "ik_end"
+    assert sanitize_username("name_service") == "name_service"
+    assert sanitize_username("@client_support") == "client_support"
+    assert sanitize_username("telegram_user") == "telegram_user"
+    assert username_for_display("ik_end", with_at=True) == "@ik_end"
+    assert username_for_display("name_service", with_at=True) == "@name_service"
+
+
+def test_sanitize_username_rejects_free_form_values_instead_of_truncating():
+    assert sanitize_username("https://t.me/name_service") is None
+    assert sanitize_username("name service") is None
+
+
+def test_display_name_filters_still_apply_to_free_form_names():
+    assert sanitize_display_name("Name service") == "Name"