ci: support release branches and match client branch (#3990)

- Update workflows to trigger on release-X.Y.x-maint branches
- When PR targets release branch, fetch matching branch from
llama-stack-client-python
- Falls back to main if matching client branch doesn't exist
- Updated workflows:
  - integration-tests.yml
  - integration-auth-tests.yml
  - integration-sql-store-tests.yml
  - integration-vector-io-tests.yml
  - unit-tests.yml
  - backward-compat.yml
  - pre-commit.yml

.github/workflows/unit-tests.yml

@@ -4,9 +4,13 @@ run-name: Run the unit test suite
 
 on:
   push:
-    branches: [ main ]
+    branches:
+      - main
+      - 'release-[0-9]+.[0-9]+.x-maint'
   pull_request:
-    branches: [ main ]
+    branches:
+      - main
+      - 'release-[0-9]+.[0-9]+.x-maint'
     paths:
       - 'src/llama_stack/**'
       - '!src/llama_stack/ui/**'