Fix Poetry resolution by using poetry init --python with narrow range

`poetry new` writes `requires-python = ">=X.Y"` (open-ended) into the
generated pyproject.toml. Poetry's resolver then has to satisfy the
project's full declared range, which currently fails:

  torchvision requires Python !=3.14.1,>=3.10, so it will not be
  installable for Python 3.14.1.
  Because no versions of torchvision match >0.27.0,<0.28.0
   and torchvision (0.27.0) requires Python !=3.14.1,>=3.10,
   torchvision is forbidden.
  So, because test-poetry depends on torchvision (^0.27.0), version
  solving failed.

3.14.1 falls inside `>=X.Y` but is excluded by torchvision
(pytorch/vision#9307), so no torchvision version is lockable.

Replace `poetry new test_poetry` with `poetry init --python
">=${MATRIX_PYTHON_VERSION},<3.$((... + 1))"` which writes a
narrow range into pyproject.toml directly. For
MATRIX_PYTHON_VERSION=3.11 the project ends up at
">=3.11,<3.12", which excludes 3.14.1 from the declared range,
so torchvision's exclusion no longer applies and resolution
succeeds.

This is also more accurate: the test now reflects the runtime
we actually set up via `conda create python=${MATRIX_PYTHON_VERSION}`,
rather than the open-ended "X.Y+" range `poetry new` defaults to.

Authored with Claude Code.

Author: atalman

.github/scripts/validate_poetry.sh

@@ -9,8 +9,15 @@ curl -sSL https://install.python-poetry.org | python3 - --version "${POETRY_VERS
 export PATH="/root/.local/bin:$PATH"
 
 poetry --version
-poetry new test_poetry
+# `poetry init` (instead of `poetry new`) so we can give the project a
+# narrow Python range. `poetry new` always writes an open-ended
+# requires-python = ">=X.Y", which breaks Poetry resolution whenever a
+# target wheel excludes a future Python that falls in the range (e.g.
+# torchvision 0.27.0 excludes Python 3.14.1 -- pytorch/vision#9307).
+mkdir test_poetry
 cd test_poetry
+poetry init --no-interaction --name test_poetry \
+    --python ">=${MATRIX_PYTHON_VERSION},<3.$((${MATRIX_PYTHON_VERSION#*.} + 1))"
 
 TEST_SUFFIX=""
 if [[ ${TORCH_ONLY} == 'true' ]]; then