refactor(python): install poetry via mise default backend (#625)

* refactor(python): switch poetry installation back to mise native

- Stop using pipx to install poetry.
- Revert to using the native mise poetry plugin, which is now preferred for better environment integration.

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* refactor(python-poetry): remove default start command from config

* Remove the explicit start command to allow for standard project-level runtime defaults.
* The previous comment regarding poetry venv path issues is addressed by updated platform base images.

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* test: update snapshot

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* chore: remove unused railpack config example

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examples/python-poetry/railpack.json

@@ -1,9 +0,0 @@
-{
-    "$schema": "https://schema.railpack.com",
-    "deploy": {
-      // without this, `python` is used directly, running `poetry` directly could fail if it was copied
-      // into the poetry venv from a previous step's binary which has since been removed from the image
-      // we ran into this originally when installing python3-dev
-      "startCommand": "poetry run python main.py"
-    }
-}