refactor(gateway): share outbound text splitting

surfsense_backend/app/gateway/base/formatting.py

@@ -0,0 +1,38 @@
+"""Provider-neutral message formatting helpers."""
+
+from __future__ import annotations
+
+MAX_GATEWAY_TEXT_CHARS = 4096
+
+
+def split_text_message(
+    text: str,
+    *,
+    max_chars: int = MAX_GATEWAY_TEXT_CHARS,
+) -> list[str]:
+    """Split outbound text at readable boundaries without exceeding platform caps."""
+    if not text:
+        return [""]
+
+    chunks: list[str] = []
+    remaining = text
+    while remaining:
+        if len(remaining) <= max_chars:
+            chunks.append(remaining)
+            break
+
+        candidate = remaining[:max_chars]
+        boundary = max(
+            candidate.rfind("\n\n"),
+            candidate.rfind("\n"),
+            candidate.rfind(". "),
+            candidate.rfind(" "),
+        )
+        if boundary <= max(200, max_chars // 2):
+            boundary = max_chars
+        split_at = boundary + (2 if candidate[boundary : boundary + 2] == ". " else 1)
+        chunk = remaining[:split_at].rstrip()
+        chunks.append(chunk or remaining[:max_chars])
+        remaining = remaining[split_at:].lstrip()
+
+    return chunks