fix(rpc): avoid retrying unavailable submissions

[huklaa]

Summary

Fixes #2441.

Transaction submission RPCs are not safe to retry on Unavailable: the node may already have accepted the first request before the response was lost, so retrying can turn a successful submission into an apparent application-level rejection.

This change makes the existing retry policy endpoint-aware:

• SubmitProvenTx and SubmitProvenBatch retry ResourceExhausted only;
• read-only RPCs keep retrying both ResourceExhausted and Unavailable;
• focused tests cover both submission-policy branches;
• the Unreleased changelog documents the behavior change.

Testing

cargo test -p miden-client rpc::tonic_client::retry --lib ✅

The branch is based on next and contains one focused commit.