refactor(tests): derive the two duplicated test helpers into tests/helpers.py (#4276)

Derive-first, ahead of the tests shape pay-down. An AST pass over all 2628
definitions under `tests/` found exactly three helper bodies duplicated
across files. Two are real copies and are consolidated here; the third is
not, and is left alone.

`need_tools(*tools)` — byte-identical in three hook-test modules, each
skipping when `jq`/`awk`/`git` is absent from PATH. Now snippet #4277. The
`import shutil` each file carried existed only to serve it and goes with it.

`rule_row(rule_id)` — byte-identical in two integration modules, reading a
Rule back through a SEPARATE session so the assertion is about what Postgres
holds rather than what the writing session's identity map remembers. Now
snippet #4278. Its imports are lazy, because `tests/helpers.py` is imported
by unit tests that have no database, which is the same reason
`plugin_config` defers its service imports.

NOT consolidated: `_side(uid, k, d="")` in test_services_plugin_context and
test_write_path_trigger. The body is identical but it closes over a
module-local `stored` dict, so it is not self-contained and "moving" it
would mean inventing a parameter neither call site wants. That is convention
plumbing — two tests independently writing the same one-line side_effect —
and it is dismissed in the ledger rather than lifted.

Worth recording for the next pass: a repeated NAME is not a family. `_row`
is defined in five modules and only two of those share a body; the other
three (test_list_rows_brief, test_calibration_stamp, test_shape_ledger)
build entirely different objects. Grouping by name would have consolidated
three things that have nothing in common.

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

tests/test_session_slippage_readout.py

@@ -32,13 +32,14 @@ from __future__ import annotations
 
 import json
 import os
-import shutil
 import subprocess
 import time
 from pathlib import Path
 
 import pytest
 
+from tests.helpers import need_tools
+
 ROOT = Path(__file__).resolve().parents[1]
 HOOKS = ROOT / "plugin" / "hooks"
 DEFS = HOOKS / "scribe_defs.sh"
@@ -47,14 +48,8 @@ RECORDER = HOOKS / "scribe_record_outcome.sh"
 HOOKS_JSON = HOOKS / "hooks.json"
 
 
-def _need(*tools):
-    for t in tools:
-        if shutil.which(t) is None:
-            pytest.skip(f"hook runtime tool {t!r} not installed")
-
-
 def sh(script: str) -> str:
-    _need("bash", "awk")
+    need_tools("bash", "awk")
     r = subprocess.run(
         ["bash", "-c", f'set -uo pipefail\n. "{DEFS}"\n{script}'],
         capture_output=True, text=True, timeout=30,
@@ -173,7 +168,7 @@ def test_a_session_no_rule_touched_says_nothing_at_all(tmp_path):
 # ── The hook that carries it ──────────────────────────────────────────────
 
 def run_precompact(event: dict, tmpdir: Path) -> subprocess.CompletedProcess:
-    _need("bash")
+    need_tools("bash")
     env = dict(os.environ)
     env["TMPDIR"] = str(tmpdir)
     return subprocess.run(["bash", str(PRECOMPACT)], input=json.dumps(event),
@@ -210,7 +205,7 @@ def test_the_compaction_hook_never_emits_a_json_envelope(tmp_path):
 # ── The recorder that makes `acted` mean anything ─────────────────────────
 
 def run_recorder(event: dict, tmpdir: Path) -> subprocess.CompletedProcess:
-    _need("bash")
+    need_tools("bash")
     env = {"PATH": os.environ["PATH"], "HOME": str(tmpdir), "TMPDIR": str(tmpdir)}
     return subprocess.run(["bash", str(RECORDER)], input=json.dumps(event),
                           capture_output=True, text=True, timeout=30, env=env)
@@ -257,7 +252,7 @@ def test_the_recorder_is_registered_on_the_rule_outcome_tool():
 
 
 def test_the_recorder_is_shell_valid():
-    _need("bash")
+    need_tools("bash")
     subprocess.run(["bash", "-n", str(RECORDER)], check=True)