core/cli: multiline \"\"\" input and /edit via EDITOR

Add triple-quote block messages in the REPL and /edit to compose a turn
in $EDITOR (temp file). Empty multiline or empty edit cancels.

tests/test_cli/test_input_text.py

@@ -0,0 +1,61 @@
+from __future__ import annotations
+
+from plyngent.cli.input_text import (
+    assemble_multiline,
+    finish_triple_quote_line,
+    parse_triple_quote_line,
+    read_repl_entry,
+)
+
+
+def test_parse_same_line_block() -> None:
+    assert parse_triple_quote_line('"""hello"""') == ("hello", True)
+    assert parse_triple_quote_line('  """hi"""  ') == ("hi", True)
+
+
+def test_parse_open_block() -> None:
+    assert parse_triple_quote_line('"""first') == ("first", False)
+    assert parse_triple_quote_line('"""') == ("", False)
+
+
+def test_parse_not_block() -> None:
+    assert parse_triple_quote_line("hello") is None
+    assert parse_triple_quote_line("/help") is None
+
+
+def test_finish_closer() -> None:
+    assert finish_triple_quote_line('"""') == ("", True)
+    assert finish_triple_quote_line('last line"""') == ("last line", True)
+    assert finish_triple_quote_line("middle") == ("middle", False)
+
+
+def test_assemble_multiline() -> None:
+    lines = iter(["line two", '"""'])
+    text = assemble_multiline(
+        '"""line one',
+        read_line=lambda: next(lines),
+    )
+    assert text == "line one\nline two"
+
+
+def test_assemble_empty_cancels() -> None:
+    lines = iter(['"""'])
+    assert assemble_multiline('"""', read_line=lambda: next(lines)) is None
+
+
+def test_read_repl_entry_slash() -> None:
+    assert read_repl_entry(read_line=lambda: "/status") == "/status"
+
+
+def test_read_repl_entry_simple() -> None:
+    assert read_repl_entry(read_line=lambda: "  hi  ") == "hi"
+
+
+def test_read_repl_entry_empty() -> None:
+    assert read_repl_entry(read_line=lambda: "   ") is None
+
+
+def test_read_repl_entry_multiline() -> None:
+    seq = iter(['"""a', "b", '"""'])
+    text = read_repl_entry(read_line=lambda: next(seq), echo=lambda _s: None)
+    assert text == "a\nb"