Bump PET version to release/2026.4 in stable pipeline

Co-authored-by: eleanorjboyd <26030610+eleanorjboyd@users.noreply.github.com>

Author: Copilot

build/azure-pipeline.stable.yml

@@ -128,7 +128,7 @@ extends:
           project: 'Monaco'
           definition: 593
           buildVersionToDownload: 'latestFromBranch'
-          branchName: 'refs/heads/release/2026.0'
+          branchName: 'refs/heads/release/2026.4'
           targetPath: '$(Build.SourcesDirectory)/python-env-tools/bin'
           artifactName: 'bin-$(buildTarget)'
           itemPattern: |