fix(deepagents): backend adapter drops route prefixes (#691)

### Summary

`adaptBackendProtocol` rebuilds the backend into a v2 backend protocol
but drops its `routePrefixes`. Since `resolveBackend` runs every backend
through the adapter, a resolved `CompositeBackend` was no longer
detected as composite. That made the execute tool's guard fail at
invocation and disable the execute tool even when filesystem permissions
were correctly scoped to composite routes.

This PR fixes this by forwarding `routePrefixes` through
`adaptBackendProtocol` instead of dropping them.

### Tests

Added unit tests in backends/utils.test.ts to verify that
`routePrefixes` is forwarded through `adaptBackendProtocol` and
`adaptSandboxProtocol` and not added for non-composite backends.