fix(channels): harden message split boundaries

tests/utils/test_helpers.py

@@ -39,6 +39,42 @@ def test_split_message_preserves_indentation_when_newline_is_at_hard_break():
     ]
 
 
+def test_split_message_handles_crlf_before_hard_break():
+    content = "header\r\n    indented code"
+
+    assert split_message(content, max_len=18) == ["header", "    indented code"]
+    assert split_message("abcdefg\r\n    code", max_len=8) == [
+        "abcdefg",
+        "    code",
+    ]
+
+
+def test_split_message_preserves_indent_after_space_then_newline_boundary():
+    content = "abcdef \n    code"
+
+    assert split_message(content, max_len=7) == ["abcdef", "    cod", "e"]
+    assert split_message(content.replace("\n", "\r\n"), max_len=7) == [
+        "abcdef",
+        "    cod",
+        "e",
+    ]
+
+
+def test_split_message_drops_blank_chunks_from_long_indentation():
+    content = "head\n" + " " * 20 + "x"
+
+    chunks = split_message(content, max_len=8)
+
+    assert chunks == ["head", "    x"]
+    assert all(chunk.strip() for chunk in chunks)
+
+
+def test_split_message_drops_whitespace_only_line_at_boundary():
+    content = "    \nhello world"
+
+    assert split_message(content, max_len=8) == ["hello", "world"]
+
+
 def test_split_message_drops_whitespace_only_tail_after_hard_break():
     prefix = "abcdefgh"
 
@@ -46,6 +82,12 @@ def test_split_message_drops_whitespace_only_tail_after_hard_break():
     assert split_message(prefix + " ", max_len=8) == [prefix]
 
 
+def test_split_message_keeps_one_chunk_for_all_whitespace_input():
+    content = " " * 10
+
+    assert split_message(content, max_len=4) == [" " * 4]
+
+
 def test_split_message_nonpositive_maxlen_returns_unsplit():
     content = "alpha beta gamma delta"