fix(generator): switch core_deps_from_source to DEFAULT_PYTHON_VERSION (#18050)

Update `core_deps_from_source` nox sessions in the noxfile template and
integration test goldens to run on `DEFAULT_PYTHON_VERSION` instead of
`PREVIEW_PYTHON_VERSION`. Running against unreleased preview Python
versions causes CI failures due to missing prebuilt wheels on PyPI for
third-party dependencies.

Author: ohmayr

.github/workflows/gapic-generator-tests.yml

@@ -133,7 +133,7 @@ jobs:
           pip install nox
           cd packages/gapic-generator
           for pkg in credentials eventarc logging redis; do
-            nox -f tests/integration/goldens/$pkg/noxfile.py -s format lint unit-${LATEST_STABLE_PYTHON}
+            nox -f tests/integration/goldens/$pkg/noxfile.py -s format lint unit-${LATEST_STABLE_PYTHON} core_deps_from_source
           done
       # Run pylint (errors-only) over the goldens so generator regressions
       # like undefined names or import-time breakage that ruff/flake8 do
@@ -171,7 +171,7 @@ jobs:
           pip install nox
           cd packages/gapic-generator
           for pkg in credentials eventarc logging redis; do
-            nox -f tests/integration/goldens/$pkg/noxfile.py -s core_deps_from_source prerelease_deps
+            nox -f tests/integration/goldens/$pkg/noxfile.py -s prerelease_deps
           done
 
   fragment-snippet:

packages/gapic-generator/gapic/templates/noxfile.py.j2

@@ -572,7 +572,7 @@ def prerelease_deps(session, protobuf_implementation):
     )
 
 
-@nox.session(python=PREVIEW_PYTHON_VERSION)
+@nox.session(python=DEFAULT_PYTHON_VERSION)
 @nox.parametrize(
     "protobuf_implementation",
     ["python", "upb"],

packages/gapic-generator/tests/integration/goldens/asset/noxfile.py

@@ -564,7 +564,7 @@ def prerelease_deps(session, protobuf_implementation):
     )
 
 
-@nox.session(python=PREVIEW_PYTHON_VERSION)
+@nox.session(python=DEFAULT_PYTHON_VERSION)
 @nox.parametrize(
     "protobuf_implementation",
     ["python", "upb"],

packages/gapic-generator/tests/integration/goldens/credentials/noxfile.py

@@ -564,7 +564,7 @@ def prerelease_deps(session, protobuf_implementation):
     )
 
 
-@nox.session(python=PREVIEW_PYTHON_VERSION)
+@nox.session(python=DEFAULT_PYTHON_VERSION)
 @nox.parametrize(
     "protobuf_implementation",
     ["python", "upb"],

packages/gapic-generator/tests/integration/goldens/eventarc/noxfile.py

@@ -564,7 +564,7 @@ def prerelease_deps(session, protobuf_implementation):
     )
 
 
-@nox.session(python=PREVIEW_PYTHON_VERSION)
+@nox.session(python=DEFAULT_PYTHON_VERSION)
 @nox.parametrize(
     "protobuf_implementation",
     ["python", "upb"],

packages/gapic-generator/tests/integration/goldens/logging/noxfile.py

@@ -564,7 +564,7 @@ def prerelease_deps(session, protobuf_implementation):
     )
 
 
-@nox.session(python=PREVIEW_PYTHON_VERSION)
+@nox.session(python=DEFAULT_PYTHON_VERSION)
 @nox.parametrize(
     "protobuf_implementation",
     ["python", "upb"],

packages/gapic-generator/tests/integration/goldens/logging_internal/noxfile.py

@@ -564,7 +564,7 @@ def prerelease_deps(session, protobuf_implementation):
     )
 
 
-@nox.session(python=PREVIEW_PYTHON_VERSION)
+@nox.session(python=DEFAULT_PYTHON_VERSION)
 @nox.parametrize(
     "protobuf_implementation",
     ["python", "upb"],

packages/gapic-generator/tests/integration/goldens/redis/noxfile.py

@@ -564,7 +564,7 @@ def prerelease_deps(session, protobuf_implementation):
     )
 
 
-@nox.session(python=PREVIEW_PYTHON_VERSION)
+@nox.session(python=DEFAULT_PYTHON_VERSION)
 @nox.parametrize(
     "protobuf_implementation",
     ["python", "upb"],

packages/gapic-generator/tests/integration/goldens/redis_selective/noxfile.py

@@ -564,7 +564,7 @@ def prerelease_deps(session, protobuf_implementation):
     )
 
 
-@nox.session(python=PREVIEW_PYTHON_VERSION)
+@nox.session(python=DEFAULT_PYTHON_VERSION)
 @nox.parametrize(
     "protobuf_implementation",
     ["python", "upb"],

packages/gapic-generator/tests/integration/goldens/storagebatchoperations/noxfile.py

@@ -564,7 +564,7 @@ def prerelease_deps(session, protobuf_implementation):
     )
 
 
-@nox.session(python=PREVIEW_PYTHON_VERSION)
+@nox.session(python=DEFAULT_PYTHON_VERSION)
 @nox.parametrize(
     "protobuf_implementation",
     ["python", "upb"],