[wic][PATCH v3 08/10] tests: ignore E402 in the test tree for the sys.path bootstrap

The unit tests that follow run against a plain checkout that has not
been installed. To do that, each test module prepends the in-tree src/
directory to sys.path and only then imports wic:

    _SRC = Path(__file__).resolve().parent.parent.parent / "src"
    if str(_SRC) not in sys.path:
        sys.path.insert(0, str(_SRC))

    from wic.bb.utils import mkdirhier

That bootstrap necessarily runs before the wic imports, which trips
ruff's E402 (module-level import not at top of file). The ordering is
required, not accidental: the import would fail without the sys.path
adjustment ahead of it.

Rather than scatter per-line noqa comments through every test module,
this commit relaxes E402 for the test tree once, via a
[tool.ruff.lint.per-file-ignores] entry scoped to tests/**. It is the
only rule relaxed for tests/; the suite is otherwise held to a clean
bar by --lint-tests.

tests/docs/linting.md gains a section describing this single exception
and why it exists.

AI-Generated: codex/claude-opus 4.7 (xhigh)
Signed-off-by: Trevor Woerner <[email protected]>
---
changes in v3:
- no change in this revision.
changes in v2:
- v1 submitted the entire test suite as a single commit; v2 breaks
  the work into a reviewable series, and this patch is one step of it.
---
 pyproject.toml        |  8 ++++++++
 tests/docs/linting.md | 20 ++++++++++++++++++++
 2 files changed, 28 insertions(+)

diff --git a/pyproject.toml b/pyproject.toml
index 656adcd4930a..fb42fe6dd135 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -58,3 +58,11 @@ tmp_path_retention_count  = 1
 # not yet ruff-clean and is left out until its findings are fixed (see
 # tests/docs/linting.md). --lint-src can still be run to preview the
 # source findings, but it is reported, not enforced.
+
+[tool.ruff.lint.per-file-ignores]
+# Test modules prepend the in-tree src/ directory to sys.path before
+# importing wic, so the suite runs against a checkout that has not been
+# installed. That bootstrap necessarily runs before the wic imports,
+# which trips E402 (module-level import not at top of file). The
+# ordering is deliberate; ignore E402 for the test tree only.
+"tests/**" = ["E402"]
diff --git a/tests/docs/linting.md b/tests/docs/linting.md
index 71b4de21c100..1c1bfcc06f82 100644
--- a/tests/docs/linting.md
+++ b/tests/docs/linting.md
@@ -4,6 +4,7 @@
 
 - [Running the linter](#running-the-linter)
 - [tests/ must be clean](#tests-must-be-clean)
+- [The one intentional exception in tests/](#the-one-intentional-exception-in-tests)
 - [src/ is not linted yet](#src-is-not-linted-yet)
 
 The test suite is linted with [ruff](https://docs.astral.sh/ruff/). It
@@ -28,6 +29,25 @@ Our own test code is held to a clean bar: `tests/run-tests.sh
 --lint-tests` reports nothing. If you add a test that trips a rule, fix
 the test before the change lands.
 
+## The one intentional exception in tests/
+
+Test modules prepend the in-tree `src/` directory to `sys.path` before
+importing `wic`, so the suite runs against a plain checkout that has not
+been installed:
+
+```python
+_SRC = Path(__file__).resolve().parent.parent.parent / "src"
+if str(_SRC) not in sys.path:
+    sys.path.insert(0, str(_SRC))
+
+from wic.bb.utils import mkdirhier
+```
+
+That bootstrap necessarily runs before the `wic` imports, which trips
+`E402` (module-level import not at top of file). The ordering is
+required, so `E402` is ignored for the test tree via `per-file-ignores`
+in `pyproject.toml`. This is the only rule relaxed for `tests/`.
+
 ## src/ is not linted yet
 
 `--lint-src` runs ruff over the wic source, but the source is **not**
-- 
2.50.0.173.g8b6f19ccfc3a