Bump PET version to 2026.4 in stable release pipeline (#25847)

Updates the Python Environment Tools (PET) artifact source branch in the
stable release pipeline.

- Changed `branchName` from `refs/heads/release/2026.0` to
`refs/heads/release/2026.4` in `build/azure-pipeline.stable.yml`

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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: |