test: add pyproject.toml to card_via_* extension packages

[talsperre]

Summary

Add a minimal pyproject.toml with the standard [build-system] table to three test-fixture packages:

• test/extensions/packages/card_via_extinit/
• test/extensions/packages/card_via_init/
• test/extensions/packages/card_via_ns_subpackage/

Each new file is 3 lines:

[build-system]
requires = ["setuptools>=61"]
build-backend = "setuptools.build_meta"

setup.py is preserved unchanged; setuptools.build_meta uses it as the metadata source.

Motivation

These three packages currently ship setup.py only — no pyproject.toml. That forces pip onto the legacy non-PEP-517 build path, which writes build/bdist.linux-x86_64/wheel/... and *.egg-info/ inside the source directory.

When CI installs them concurrently from the same source tree (e.g. multiple tox environments running in parallel installing them all as deps), the concurrent pip processes race on those in-source directories and produce errors like:

[Errno 2]  No such file or directory: 'build/bdist.linux-x86_64/wheel/./metaflow_card_via_extinit-1.0.0-py3.10.egg-info/dependency_links.txt'
[Errno 17] File exists:    'build/bdist.linux-x86_64/wheel/metaflow_card_via_extinit-1.0.0.dist-info'
[Errno 39] Directory not empty: 'build/bdist.linux-x86_64/wheel'
[Errno 2]  No such file or directory: 'metaflow_card_via_nspackage.egg-info/tmp8cck71cx'

Adding the [build-system] table switches pip to the PEP 517 isolated-build path: pip copies the source tree to a unique /tmp/pip-build-env-*/ per install, and the wheel build runs there. Concurrent installs no longer share mutable state in the source directory, and the race goes away.

Test plan

• Local build still works: pip wheel -w /tmp/wh ./test/extensions/packages/card_via_extinit/ produces metaflow_card_via_extinit-1.0.0-py3-none-any.whl
• The legacy python setup.py bdist_wheel invocation still works (setuptools' build_meta backend is backed by setup.py)
• No source-dir mutation after a PEP 517 install (verified build/ and *.egg-info/ do not appear in source after install)

[greptile-apps]

Greptile Summary

This PR adds a minimal pyproject.toml to three test-fixture packages (card_via_extinit, card_via_init, card_via_ns_subpackage) to opt them into the PEP 517 isolated-build path, fixing a CI race condition that occurs when multiple pip processes install from the same source tree concurrently.

• Each new file is identical: a [build-system] table declaring setuptools>=61 and setuptools.build_meta, which causes pip to copy the source to a unique temp directory before building rather than writing build/ and *.egg-info/ artifacts into the shared source tree.
• The existing setup.py files are untouched and continue to serve as the metadata source under setuptools.build_meta; all three packages also have README.md present, so the open(\"README.md\") call in each setup.py will resolve correctly in the isolated build environment.

Confidence Score: 5/5

Safe to merge — the change is limited to three test-fixture packages and does not touch any production code or CI configuration.

Three identical, three-line TOML files are added to test-only packages. Each file is a well-established, standard [build-system] declaration. The existing setup.py files and README.md files are untouched and remain compatible with the new build path. There is no risk to production behaviour.

No files require special attention.

Important Files Changed

Filename	Overview
test/extensions/packages/card_via_extinit/pyproject.toml	New minimal pyproject.toml enabling PEP 517 isolated builds for the card_via_extinit test fixture package.
test/extensions/packages/card_via_init/pyproject.toml	New minimal pyproject.toml enabling PEP 517 isolated builds for the card_via_init test fixture package.
test/extensions/packages/card_via_ns_subpackage/pyproject.toml	New minimal pyproject.toml enabling PEP 517 isolated builds for the card_via_ns_subpackage test fixture package.

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