Let the output "x" delete work when no model/session exists (#1431)

deleteMessage() bailed at `if (!sessionId) return;`, so the "x" on an output
shown before a model/API was selected did nothing — there's no session yet
(issue #1428). The session id is only needed for the server-side delete; without
one (or with no persisted message ids) we now fall through to removing the DOM,
so the "x" always at least dismisses the bubble.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_delete_message_no_session.py

@@ -0,0 +1,34 @@
+"""Regression guard for issue #1428 — the "x" on a chat output did nothing when
+no model/API was selected.
+
+deleteMessage() bailed at `if (!sessionId) return;`. An output shown before a
+model is picked has no session and no persisted rows, so the early-out meant the
+"x" never even removed the bubble from the DOM. The delete now falls through to
+DOM removal when there's no session / no DB ids.
+
+chat.js pulls in browser globals so it can't run under node; guard at the source.
+"""
+import re
+from pathlib import Path
+
+SRC = Path(__file__).resolve().parent.parent / "static/js/chat.js"
+
+
+def _delete_message_body() -> str:
+    text = SRC.read_text(encoding="utf-8")
+    start = text.index("export async function deleteMessage(")
+    rest = text[start:]
+    m = re.search(r"\n  export (async )?function ", rest[1:])
+    return rest[: m.start() + 1] if m else rest
+
+
+def test_delete_does_not_early_return_on_missing_session():
+    body = _delete_message_body()
+    # The bug was an unconditional early-out when no session existed.
+    assert not re.search(r"if\s*\(\s*!sessionId\s*\)\s*return\s*;", body), (
+        "deleteMessage must not early-return on a missing session (#1428)"
+    )
+    # The DOM-removal fallback must also fire when there's no session.
+    assert re.search(r"!msgIds\.length\s*\|\|\s*!sessionId", body), (
+        "DOM-removal fallback should cover the no-session case"
+    )