Add a partial bulk upsert primitive to the RBAC write ops

## Goal

Add `bulk_upsert_scoped_partial` to the RBAC write ops: the upsert counterpart of `bulk_create_scoped_partial` / `bulk_purge_scoped_partial`. Each row and its scope association share one savepoint, so a rejected row rolls back alone and the rest of the batch lands. The write ops currently offer upsert only as the single-item `upsert_scoped`.

## Acceptance criteria

- A batch with one rejected row (e.g. an FK gate violation) upserts the rest; the rejected row leaves neither the row nor its scope association behind.
- The result reports successes and per-index errors, mirroring the create/purge partial primitives.

## Related

- BA-7250 consumes this primitive for partial success in the app config fragment bulk upsert.
- BA-6628 introduced the conditional-bulk partial primitives this mirrors.

JIRA Issue: BA-7256

Locate the RBAC write ops named bulk_create_scoped_partial, bulk_purge_scoped_partial, and upsert_scoped, then compare their savepoint and result-handling behavior. Add the bulk_upsert_scoped_partial entry point with per-row rollback and indexed errors, and verify that a rejected row leaves no row or scope association while other rows succeed.

Written by the indexing model from the issue text.