test(streaming): exercise an echo that really falls outside the search window

The case labelled as an echo further back than the window still ended the
buffer with the suffix, so it matched regardless of the window. Replace it
with a buffer whose whitespace-padded echo spans more than the default
window, and add a dedicated test proving the same echo is rejected with the
default window and found once the window covers it, for both the optimised
scan and the reference oracle.

tests/test_compact_echo_suffix_linear_scan.py

@@ -74,8 +74,9 @@ def _cases():
         ("aaaa", "aa"),
         # Long buffer with the echo at the very end.
         ("remplissage " * 500 + "la vraie conclusion", "la vraie conclusion"),
-        # Echo further back than the search window allows.
-        ("z" * 9000 + " tail", "tail"),
+        # Echo whose whitespace-padded span is wider than the search window:
+        # the window only exposes ``conclusion``, so nothing may be stripped.
+        ("bla une" + " " * 5000 + "conclusion", "une conclusion"),
         # Exotic whitespace that ``\s`` and ``str.isspace`` must treat alike.
         ("bla\u00a0bla une conclusion", "une\u00a0conclusion"),
         ("bla\u2028une conclusion", "une conclusion"),
@@ -123,6 +124,25 @@ def test_strip_compact_echo_suffix_honours_a_custom_search_window():
         ), f"window={window}"
 
 
+def test_strip_compact_echo_suffix_rejects_an_echo_wider_than_the_window():
+    """An echo that only fits inside a wider window must not be found.
+
+    The buffer ends with the echo, but interior whitespace stretches its raw
+    span past the default window, so the folded window only exposes the last
+    word. Widening the window to cover the whole span makes the same echo
+    visible again, which proves the non-match is caused by the window and
+    not by the text.
+    """
+    import api.streaming as streaming
+
+    buffer_text = "bla une" + " " * 5000 + "conclusion"
+    suffix = "une conclusion"
+
+    for impl in (streaming._strip_compact_echo_suffix, _reference_strip_compact_echo_suffix):
+        assert impl(buffer_text, suffix) == (buffer_text, False), impl.__name__
+        assert impl(buffer_text, suffix, search_window=len(buffer_text)) == ("bla", True), impl.__name__
+
+
 def test_strip_compact_echo_suffix_does_not_refold_the_window_per_cut(monkeypatch):
     """One pass, not one fold per candidate cut index.