fix(rust-client): use delegated (remote) proving in the delegated-proving tutorial

[Keinberger]

Overview

The "Delegated Proving" tutorial (delegated_prover.rs + delegated_proving_tutorial.md) is supposed to teach delegated proving, but it actually proved locally: it built a LocalTransactionProver, and the doc even admitted "to keep this tutorial runnable without external services, the code below uses a local prover." Miden runs a public testnet delegated prover, so the tutorial can just do the real thing.

This PR points the tutorial at the hosted testnet delegated prover so it performs actual delegated (remote) proving.

Key Changes

• rust-client/src/bin/delegated_prover.rs: build RemoteTransactionProver::new("https://tx-prover.testnet.miden.io") instead of LocalTransactionProver, and drop it into the existing prove_transaction_with(...) call. Both implement TransactionProver, so the execute, prove, submit, apply flow is unchanged. Updated the "local prover" comment/print strings to say delegated.
• docs/src/rust-client/delegated_proving_tutorial.md: the same change inside the rust no_run block, corrected the prose that said the tutorial uses a local prover, and replaced the expected-output transcript, which was wrong (it showed account-balance lines this binary never prints) with the real nonce output.

Rationale

No new dependency: RemoteTransactionProver already ships with the miden-client the tutorial depends on (it is the same type ClientBuilder::for_testnet() uses). I used the URL as a string literal rather than the TESTNET_PROVER_ENDPOINT constant so a reader can see the endpoint and knows they can point it at their own prover. The privacy caveat (delegated proving reveals the transaction inputs, so it is not appropriate for the hash-preimage note tutorial) stays.

Tests

Ran delegated_prover against testnet: the remote prover generated the proof and Alice's nonce went 0 to 1 (exit 0). cargo test --doc (17 passed, 0 failed) and prettier --check are green.

[Keinberger]

@brianseong could you give this one a review when you get a chance? 🙏

[BrianSeong99]

Approved 👍