test: Add presubmit to check cross package compatibility

.github/workflows/test_cross_package_compatibility.yml

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+# Copyright 2025 Google LLC
+
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+
+#     https://www.apache.org/licenses/LICENSE-2.0
+
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+name: A workflow for testing cross package compatibility using pex
+
+# Controls when the action will run.
+
+# Run this action anytime `gapic_version.py` is updated in main
+on:
+  push:
+    paths:
+      - '**/gapic_version.py'
+    branches:
+      - main
+  # TODO: Remove this before merging
+  pull_request:
+    branches:
+      - main
+
+
+permissions:
+  contents: read
+
+jobs:
+  build:
+    permissions:
+      issues: write # to open issues when compatibility issues are found
+    name: Cross Package Compatibility using Pex
+    runs-on: ubuntu-latest
+    # don't run the workflow on forks of googleapis/google-cloud-python
+    if: ${{github.repository == 'googleapis/google-cloud-python'}}
+    steps:
+      - uses: actions/checkout@v4
+      - name: Set up Python 3.13
+        uses: actions/setup-python@v5
+        with:
+          python-version: 3.13
+      - name: Install build dependencies
+        run: pip install build wheel
+      - name: Build wheels for all packages
+        run: ./build_wheels.sh
+        working-directory: ./scripts/test_cross_package_compatibility
+      - name: Create requirements.txt file to install all packages from source
+        run: ./create_requirements_file.sh
+        working-directory: ./scripts/test_cross_package_compatibility
+      - name: Create python file to be used during the compatibility check
+        run: ./generate_test_code.sh
+        working-directory: ./scripts/test_cross_package_compatibility
+      - name: Install pex
+        run: pip install pex
+      - name: Install all packages and run compatibility check
+        run: pex -r requirements.txt -- test_imports.py
+        working-directory: ./scripts/test_cross_package_compatibility      
+      # TODO : open github issue if there are failures

scripts/test_cross_package_compatibility/build_wheels.sh

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+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+
+SCRIPT_ROOT=$(realpath $(dirname "${BASH_SOURCE[0]}"))
+REPO_ROOT=${SCRIPT_ROOT/%'/scripts/test_cross_package_compatibility'}
+
+# Build wheels for all packages
+for package in $REPO_ROOT/packages/*/ ; do
+    pushd ${package}
+    distribution_name=$(basename $package)
+    python3 -m build --wheel
+    popd
+done

scripts/test_cross_package_compatibility/create_requirements_file.sh

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+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+
+SCRIPT_ROOT=$(realpath $(dirname "${BASH_SOURCE[0]}"))
+REPO_ROOT=${SCRIPT_ROOT/%'/scripts/test_cross_package_compatibility'}
+
+output=''
+# Create a requirements.txt file with all packages in this repo
+for package in $REPO_ROOT/packages/*/ ; do
+    distribution_name=$(basename $package)
+    # Replace `-` with `_`
+    distribution_name_wheel=$(echo ${distribution_name} | sed -E 's/\-/_/g')
+    output+="${distribution_name} @ file://localhost${REPO_ROOT}/packages/${distribution_name}/dist/${distribution_name_wheel}-0.0.0-py3-none-any.whl\n"
+done
+output+="# Remove pin below once https://github.com/googleapis/google-cloud-python/issues/13874 is fixed\n"
+output+="protobuf<6"
+
+# Save the output to a requirements.txt
+echo -e $output > requirements.txt

scripts/test_cross_package_compatibility/generate_test_code.sh

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+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR from google.cloud.billing import CloudBillingClientDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+
+SCRIPT_ROOT=$(realpath $(dirname "${BASH_SOURCE[0]}"))
+REPO_ROOT=${SCRIPT_ROOT/%'/scripts/test_cross_package_compatibility'}
+
+output=''
+# Create a requirements.txt file with all packages in this repo
+for package in $REPO_ROOT/packages/*/ ; do
+    package_dir=$(basename $package)
+    top_namespace="${package_dir%%-*}"
+    for d in `find $package$top_namespace -name '__init__.py'`; do
+        init_relative_path=${d#$package}
+        module_path=$(echo $init_relative_path | sed -e "s/\/__init__.py//")
+        import_path="$(echo $module_path | sed -E 's/\//./g')"
+        output+="from $import_path import *\n"
+    done    
+done
+# Save the output to a python file
+echo -e $output > test_imports.py