Isolate each call of a function decorated with a MockVWS instance - #3453

`MockVWS` inherited from `ContextDecorator`, so every use of one
instance shared a single set of databases and targets. Decorating two
test functions with one instance therefore made them affect each other,
which is the opposite of what the decorator idiom implies and of what
`@responses.activate` and `@mock.patch` do.

Each call of a decorated function now gets its own target manager and
its own API fakes. The databases added to the instance are available
inside the call, and, because targets are stored on the caller's own
database object, their targets are snapshotted and restored around the
call so that a database can still be inspected while the call runs.

Using an instance as a context manager is unchanged: a `with` block
still shares its state with every other use of the same instance.

Along the way:

- Replace the constructor arguments held for this purpose with a frozen
  `_MockVWSOptions` dataclass, so the options are read as typed
  attributes rather than unpacked from a `dict[str, Any]`.
- Keep the started mocks as a stack, so that a decorated function
  calling another decorated function no longer leaves the outer call's
  mocking stopped.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>