docs(ntx-builder): clarify network note FPI support

[Mr-kay-cloud2]

Summary

• Documents network-note FPI support for public foreign accounts.
• Clarifies that the foreign account must be public and available from the store at execution time.
• Documents how missing/private/unavailable foreign account failures appear through GetNetworkNoteStatus.last_error.
• Adds source comments around the ntx-builder foreign account fetch path.

Closes #2046

Testing

• rustfmt +nightly --edition 2024 bin/ntx-builder/src/actor/execute.rs bin/ntx-builder/src/clients/store.rs
• cargo check -p miden-ntx-builder --all-targets could not run locally on Windows because the miden-agglayer dependency checkout fails on an invalid : path in a submodule file.