[edmorley]

Ports the applicable editable fixture improvements from the classic buildpack (heroku/heroku-buildpack-python#2155) to the CNB's pip_editable_git_compiled, poetry_editable_git_compiled and uv_editable_git_compiled fixtures and tests:

• Every fixture now installs the current project in editable mode, using the package manager's default build backend and src layout. Previously only uv did, the pip fixture had only the VCS requirement, and the Poetry fixture used package-mode = false.
• Each test now checks at launch-time that the project's entrypoint script is on PATH and runs from the app source directory (so the install really is editable), and that the compiled VCS package is imported from the expected location. Previously only the Poetry test checked anything at launch-time.
• The extension.dist VCS dependency is updated from wheel 0.44.0 to 0.45.1 (the newest tag that still contains it), to match the classic fixtures.

The Poetry test now also expects the informational setuptools BuildBackendWarning that's emitted when installing the flat-layout VCS package in editable mode, since the new assertion continues past it (the old one ended just before it).

Unlike the classic buildpack, the CNB doesn't rewrite editable install paths (the build and launch paths are identical), so the classic fixtures' additional build backend/layout packages and path rewriting checks haven't been ported.

In addition:

• The testing buildpack and launch-time check output is now labelled (for example Build-time env vars:, Python sys.path: and typing_extensions imported from: ...), so that the asserted output is easier to understand.
• "run-time" in comments has been changed to "launch-time", to match the CNB spec's terminology, and the fixture and test comments have been made consistent.

GUS-W-24469384.