digest: factor tone clause; kw-only digest helper; empty-data test

tests/test_digest_prompts.py

@@ -46,3 +46,19 @@ def test_prompts_return_strings():
         sys_, usr = fn(tone="INTERMEDIATE", **_ctx())
         assert isinstance(sys_, str) and isinstance(usr, str)
         assert len(sys_) > 50 and len(usr) > 50
+
+
+def test_prompts_tolerate_empty_data():
+    """No quotes, no headlines — builders must still produce non-empty
+    prompts without raising. Guards the `if headlines_by_bucket` and
+    `if quotes_by_group` branches in _digest_user_prompt."""
+    empty_ctx = dict(
+        today=datetime(2026, 5, 25, 6, 30, tzinfo=timezone.utc),
+        quotes_by_group={},
+        headlines_by_bucket={},
+        reference_line="S&P 7,501 (ATH)",
+    )
+    for fn in (build_daily_digest_prompt, build_weekly_digest_prompt):
+        sys_, usr = fn(tone="INTERMEDIATE", **empty_ctx)
+        assert "S&P 7,501" in usr
+        assert len(sys_) > 50