buildkite: consolidate lints into monorepo-integrity-tests (#25258)

## Summary & Motivation

The `buildkite-lints` Buildkite step (`dagster-oss/.buildkite/buildkite-shared/lints.py`) was running five pytest-style checks on every PR. Three of those checks already exist verbatim in `integration_tests/monorepo-integrity-tests/conventions_tests/` — `test_tox_passenv`, `test_no_subdirectory_pytest_ini`, `test_no_tox_pytest_config_override` — and the `conventions_tests` versions run repo-wide via `git ls-files` rather than the dagster-oss-only `os.walk(".")` scan, so they're a strict superset. The remaining two — `test_tox_has_lockfile` and `test_pyproject_has_requires_python` — had no counterpart in `conventions_tests` despite being conceptually identical in spirit.

This PR moves the two unique checks into `conventions_tests/` and deletes `lints.py` and the `buildkite-lints` step entirely, consolidating all pytest-shaped repo-wide checks into one place.

## Changes

- `conventions_tests/test_tox_and_pytest_configuration.py`: added `test_dagster_oss_tox_has_lockfile`. Refactored the existing fixtures so `tox_config` now depends on a new `tox_ini_path` fixture, and added a sibling `dagster_oss_tox_ini_path` fixture (scoped to `dagster-oss/**/tox.ini`) that the new test consumes. Inlined the `_is_gitignored` helper for the uv.lock opt-out path.
- `conventions_tests/test_package_structure.py`: added `test_dagster_oss_project_pyprojects_have_requires_python`, following the aggregated-violations pattern already used by `test_all_oss_packages_have_py_typed` in that file.
- Deleted `dagster-oss/.buildkite/buildkite-shared/lints.py`.
- Removed `build_buildkite_lint_steps()` and its call in `build_repo_wide_steps()` from `dagster-oss/.buildkite/dagster-buildkite/dagster_buildkite/steps/dagster.py`.

## Scope of the moved tests

Both moved tests are scoped to `dagster-oss/` to match the prior `lints.py` behavior — they're not expanded to the whole internal repo. Expanding would require fixing ~30 internal pyprojects that lack `requires-python` and most internal toxfiles that aren't on `uv-venv-lock-runner`; that's a much larger separate cleanup.

## Test Plan

- `cd integration_tests/monorepo-integrity-tests && tox -e py312-conventions` — 454 tests pass (6.7s), including the two newly-added ones across all dagster-oss toxfiles and pyprojects.
- `just ruff` clean.
- Grep confirmed no other references to `lints.py`, `buildkite-lints`, or `build_buildkite_lint_steps` in the repo (internal `.buildkite/`, dagster-oss `.buildkite/`, `integration_tests/`).

Internal-RevId: d53c5c53211517f6af3f9bf90d35265ba2429741

Author: smackesey

.buildkite/buildkite-shared/lints.py

@@ -30,27 +30,13 @@
 from dagster_buildkite.steps.test_project import build_test_project_steps
 
 
-def build_buildkite_lint_steps() -> list[CommandStepConfiguration]:
-    commands = [
-        f"cd {oss_path('.')}",
-        "pytest .buildkite/buildkite-shared/lints.py",
-    ]
-    return [
-        CommandStepBuilder("buildkite-lints", [":lint-roller:"])
-        .run(*commands)
-        .on_test_image()
-        .build()
-    ]
-
-
 def build_repo_wide_steps(ctx: BuildkiteContext) -> list[StepConfiguration]:
     # Other linters may be run in per-package environments because they rely on the dependencies of
     # the target. `ty` and `ruff` are run for the whole repo at once.
     return [
         *build_repo_wide_ty_steps(ctx),
         *build_repo_wide_ruff_steps(ctx),
         *build_repo_wide_prettier_steps(ctx),
-        *build_buildkite_lint_steps(),
     ]