LookupResources Cursor section tampering can crash SpiceDB process via tuple.MustParse panic

Summary

## Description A malformed or tampered-with LookupResources [Cursor token](https://buf.build/authzed/api/docs/main:authzed.api.v1#authzed.api.v1.Cursor) can cause a panic in the SpiceDB process if it fails to parse. If an attacker were able to make requests to a SpiceDB instance, they could affect its availability. ## Reproduction If one was to take a cursor from a LookupResources call, decode it according to the logic that SpiceDB uses, and modify the Sections field to include an invalid relationship string, the process will panic. ## Impact An attacker would need both the ability to create a gRPC connection to your SpiceDB instance and a valid token, or else the ability to pass a cursor token from outside your application through to your SpiceDB instance. If an attacker had this ability, they could bring down SpiceDB instances, reducing the availability of SpiceDB and any service that depends on it. ## Mechanism The SpiceDB process does not validate the contents of this `Sections` component of the `Cursor` message. In affected versions, it uses a parsing function that calls `panic` if the value cannot be parsed as a relationship. ## Fix This issue was fixed in https://github.com/authzed/spicedb/pull/2878. ## Remediations * Prevent client control of the `optional_cursor` field in `LookupResources` calls * Upgrade to an unaffected version