fix(chat): make resend message non-destructive

Keep normal resend from truncating session history while preserving replace-from-here behavior for regenerate flows.

tests/test_resend_message_nondestructive.py

@@ -0,0 +1,43 @@
+"""Regression guard for #4149: normal Resend must not delete chat history.
+
+chat.js is browser-heavy, so this pins the source-level contract: the footer's
+plain "Resend message" path appends a fresh send, while regenerate-only paths
+must opt into truncating/replacing from the selected message.
+"""
+
+from pathlib import Path
+
+
+_REPO = Path(__file__).resolve().parent.parent
+_CHAT_JS = _REPO / "static" / "js" / "chat.js"
+_CHAT_RENDERER_JS = _REPO / "static" / "js" / "chatRenderer.js"
+
+
+def _resend_body() -> str:
+    src = _CHAT_JS.read_text(encoding="utf-8")
+    start = src.index("export async function resendUserMessage(")
+    end = src.index("export async function regenerateFrom(", start)
+    return src[start:end]
+
+
+def test_resend_message_does_not_truncate_by_default():
+    body = _resend_body()
+
+    assert "opts = {}" in body
+    assert "const replaceFromHere = Boolean(opts && opts.replaceFromHere);" in body
+
+    guard_idx = body.index("if (replaceFromHere)")
+    truncate_idx = body.index("/api/session/${sessionId}/truncate")
+    hide_idx = body.index("_hideUserBubble = true;")
+
+    assert guard_idx < truncate_idx
+    assert guard_idx < hide_idx
+    assert "/truncate" not in body[:guard_idx]
+    assert "_hideUserBubble = true;" not in body[:guard_idx]
+
+
+def test_only_regenerate_callers_opt_into_replace_from_here():
+    renderer = _CHAT_RENDERER_JS.read_text(encoding="utf-8")
+
+    assert "window.chatModule.resendUserMessage(msgElement);" in renderer
+    assert "window.chatModule.resendUserMessage(userMsgEl, { replaceFromHere: true });" in renderer