Add docs-tests harness coverage for Network Accounts snippets

[BrianSeong99]

Context

The new Network Accounts page documents deploying a network account under v0.15 (AuthNetworkAccount + NetworkAccountNoteAllowlist, replacing the removed AccountStorageMode::Network).

Its Rust snippets are grounded in — and lifted near-verbatim from — miden-client's shipped integration test bin/integration-tests/src/tests/network_transaction.rs (v0.15.2) and miden-standards v0.15.3, so they compile upstream. They are not yet covered by the local docs-tests validation harness.

Task

Add a crates/network-account-demo to the docs-tests harness that lifts the page's snippets verbatim (wrapped in exported fns) and goes green under cargo build --keep-going, so future v0.16+ API drift on:

• AuthNetworkAccount::with_allowed_notes / with_allowed_tx_scripts
• AccountBuilder + AuthNetworkAccount + BasicWallet build path
• client add_account / submit_new_transaction deploy flow
• P2idNote::script_root() / MintNote::script_root() / NoteScriptRoot

is caught before it reaches published docs.

Out of scope

• The page content itself (already authored/reviewed).
• The tutorials-repo divergence (network-transactions tutorial still references AccountStorageMode::Network) — tracked separately.