Address Copilot review: benchmark skip, pytest install, artifact guard

- Add --benchmark-skip to CI pytest runs so benchmarks only run in the
  dedicated Benchmark stage, not on every PR across all Python versions
- Explicitly install pytest in the Benchmark job instead of relying on
  transitive dependency from pytest-benchmark
- Guard PublishPipelineArtifact with a file existence check so a missing
  benchmark.json does not mask the underlying failure

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: RyAuld

.Pipelines/template-pipeline-stages.yml

@@ -197,7 +197,7 @@ stages:
         pip install pytest pytest-azurepipelines
         mkdir -p test-results
         set -o pipefail
-        pytest -vv --junitxml=test-results/junit.xml --deselect tests/test_cryptography.py::CryptographyTestCase::test_ceiling_should_be_latest_cryptography_version_plus_three --deselect tests/test_cryptography.py::CryptographyTestCase::test_should_be_run_with_latest_version_of_cryptography 2>&1 | tee test-results/pytest.log
+        pytest -vv --benchmark-skip --junitxml=test-results/junit.xml --deselect tests/test_cryptography.py::CryptographyTestCase::test_ceiling_should_be_latest_cryptography_version_plus_three --deselect tests/test_cryptography.py::CryptographyTestCase::test_should_be_run_with_latest_version_of_cryptography 2>&1 | tee test-results/pytest.log
       displayName: 'Run tests'
       retryCountOnTaskFailure: 3
       env:

azure-pipelines.yml

@@ -53,7 +53,7 @@ stages:
 
     - script: |
         python -m pip install --upgrade pip
-        pip install -r requirements.txt
+        pip install pytest -r requirements.txt
       displayName: 'Install dependencies'
 
     - task: Cache@2
@@ -66,9 +66,14 @@ stages:
         pytest --benchmark-only --benchmark-json benchmark.json --log-cli-level INFO tests/test_benchmark.py
       displayName: 'Run benchmarks'
 
+    - bash: |
+        [ -f benchmark.json ] && echo "##vso[task.setvariable variable=benchmarkFileExists]true"
+      displayName: 'Check benchmark output exists'
+      condition: succeededOrFailed()
+
     - task: PublishPipelineArtifact@1
       displayName: 'Publish benchmark results'
-      condition: succeededOrFailed()
+      condition: and(succeededOrFailed(), eq(variables['benchmarkFileExists'], 'true'))
       inputs:
         targetPath: 'benchmark.json'
         artifact: 'benchmark-results'