ci: run backend tests for provider bundle changes (#14606)

The `python` path filter never learned about `src/bundles/**` after the
bundle metapackage split. Since #13614 gated `test-backend` on
`path-filter.outputs.python == 'true'`, a PR that only touches a provider
bundle reports `python=false` and skips the entire backend suite -
including the bundle's own tests under `src/bundles/*/tests/`.

Add `src/bundles/**` to the `python` filter so bundle-only PRs run the
backend suite (and `test-templates`, which shares the same output).