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_contract_around_the_change.py

@@ -34,24 +34,19 @@ A FIXTURE REPO, NEVER THIS ONE. Asserting against Scribe's own files would
 make the test a description of today's tree, failing the next time someone
 renames something (rule 115's reasoning, one floor down).
 """
-import shutil
 import subprocess
 from pathlib import Path
 
 import pytest
 
+from tests.helpers import need_tools
+
 DEFS = Path(__file__).resolve().parents[1] / "plugin" / "hooks" / "scribe_defs.sh"
 HOOK = Path(__file__).resolve().parents[1] / "plugin" / "hooks" / "scribe_prior_art.sh"
 
 
-def _need(*tools):
-    for t in tools:
-        if shutil.which(t) is None:
-            pytest.skip(f"hook runtime tool {t!r} not installed")
-
-
 def run(script: str) -> str:
-    _need("bash", "awk", "git", "grep", "sed")
+    need_tools("bash", "awk", "git", "grep", "sed")
     r = subprocess.run(
         ["bash", "-c", f'set -uo pipefail\n. "{DEFS}"\n{script}'],
         capture_output=True, text=True,
@@ -63,7 +58,7 @@ def run(script: str) -> str:
 @pytest.fixture()
 def repo(tmp_path):
     """A small git repo: a definition with a reader, and one without."""
-    _need("git")
+    need_tools("git")
     # `other` lives in lib.py BESIDE widget, and nothing references it. That
     # placement is the point of the no-readers case: putting it in its own
     # file would leave that file as its reader, since only the file being
@@ -248,6 +243,6 @@ def test_the_hook_asks_the_contract_question_first():
 
 
 def test_the_hook_is_still_shell_valid():
-    _need("bash")
+    need_tools("bash")
     subprocess.run(["bash", "-n", str(HOOK)], check=True)
     subprocess.run(["bash", "-n", str(DEFS)], check=True)