fix(tests): the no-block guard reads the shell, not the comment explaining it (#3680)

The hook's header quotes `{"decision":"block"}` while explaining why this hook
must never emit one, and the instructions it prints use "decision" in a
sentence. Grepping the whole file caught both and failed the guard on the file
doing its job.

Strips comments and the heredoc first, and guards the stripper: `_code()`
returning nothing would make all three static checks pass against an empty
string, which is the circularity rule 167 is about.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01821k5B3Ysecp9fNYs92Kuy

tests/test_precompact_hook.py

@@ -51,6 +51,39 @@ def _run(event: dict) -> subprocess.CompletedProcess:
                           capture_output=True, text=True, timeout=30)
 
 
+def _code() -> str:
+    """The script with its comments and its heredoc removed.
+
+    The static checks below are about what the shell DOES. Run over the whole
+    file they also read the header — which quotes `{"decision":"block"}` in the
+    course of explaining why this hook must never emit one — and the emitted
+    instructions, which use the word "decision" in a sentence. Both are the
+    file doing its job, and neither is code.
+    """
+    lines, in_heredoc = [], False
+    for line in HOOK.read_text().splitlines():
+        if in_heredoc:
+            in_heredoc = line.strip() != "EOF"
+            continue
+        if line.lstrip().startswith("cat <<'EOF'"):
+            in_heredoc = True
+            continue
+        if not line.lstrip().startswith("#"):
+            lines.append(line)
+    return "\n".join(lines)
+
+
+def test_the_comment_stripper_still_leaves_the_shell_behind():
+    """Guard on the three checks below it. `_code()` returning nothing would
+    make every one of them pass while checking an empty string — the same
+    circularity the plugin version check has to defend against."""
+    code = _code()
+    assert "set -uo pipefail" in code and "exit 0" in code
+    # It really did strip: both of the words the checks look for are present
+    # in the file, and neither is in the code.
+    assert "decision" in HOOK.read_text()
+
+
 @pytest.mark.parametrize("trigger", ["manual", "auto"])
 def test_it_exits_zero_with_instructions_on_stdout(trigger):
     """Exit 0 plus non-empty stdout is the entire contract for reaching the
@@ -78,9 +111,9 @@ def test_it_emits_text_and_not_a_json_envelope():
 def test_it_never_blocks_the_compaction():
     """A block skips compaction silently from the model's side. Nothing in the
     script may produce one — not an exit code, not a decision."""
-    body = HOOK.read_text()
-    assert '"decision"' not in body and "'decision'" not in body
-    assert "exit 2" not in body
+    code = _code()
+    assert "decision" not in code, "a block decision would skip the compaction"
+    assert "exit 2" not in code
     # A truncated or absent event must not turn into a non-zero exit either.
     for event in ("", "not json", "{}"):
         out = subprocess.run(["bash", str(HOOK)], input=event,
@@ -92,7 +125,7 @@ def test_additional_context_is_not_how_this_event_works():
     """SessionStart's channel, which does not exist on PreCompact. A rewrite
     that reaches for it would read as consistent with the other hooks and
     inject nothing at all."""
-    assert "additionalContext" not in HOOK.read_text().split("set -uo pipefail")[-1]
+    assert "additionalContext" not in _code()
 
 
 def test_the_plugin_registers_it_on_precompact():