ci: trigger azure-sdk-for-python regeneration after http-client-python publish

Add a Python_Notify stage to the http-client-python publish pipeline
that dispatches a typespec-python-regenerate event to
Azure/azure-sdk-for-python after successful publish. Uses
login-to-github.yml to mint a GitHub App token via Azure Key Vault.

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

Author: iscai-msft

packages/http-client-python/eng/pipeline/publish.yml

@@ -32,3 +32,24 @@ extends:
           EnableCadlRanchReport: false
           PythonVersion: "3.11"
           UploadPlaygroundBundle: true
+
+      - stage: Python_Notify
+        displayName: Python - Notify downstream
+        dependsOn: Python_Publish
+        condition: succeeded()
+        pool:
+          name: $(LINUXPOOL)
+          image: $(LINUXVMIMAGE)
+          os: linux
+        jobs:
+          - job: DispatchRegeneration
+            displayName: Trigger azure-sdk-for-python regeneration
+            steps:
+              - template: /eng/common/pipelines/templates/steps/login-to-github.yml
+              - script: |
+                  curl -f -X POST \
+                    -H "Accept: application/vnd.github+json" \
+                    -H "Authorization: Bearer $(GH_TOKEN)" \
+                    https://api.github.com/repos/Azure/azure-sdk-for-python/dispatches \
+                    -d '{"event_type":"typespec-python-regenerate","client_payload":{"sha":"$(Build.SourceVersion)"}}'
+                displayName: Dispatch typespec-python-regenerate