Migrate gRPC schema to miden-protobuf

[Mirko-von-Leipzig]

Summary

Replace the node’s custom protobuf conversion macros and TryFrom implementations with miden-protobuf decoding, verification, and construction traits.

• Move node-specific domain conversions into miden-node-proto, including TransactionInputs and AuthenticatedTransaction.
• Consolidate manual response decoding into trait implementations.
• Verify incoming block signatures and parent linkage before applying block effects.
• Reject inconsistent account details, duplicate or missing authentication nullifiers, and mismatched account IDs.
• Preserve field context and error causes in gRPC responses, and enforce request limits before decoding.
• Update schema annotations and document unchecked construction, caller verification responsibilities, and remaining upstream limitations.

Notably, this also surfaced some problems/gaps in our verification steps. I've added SAFETY comments, some of which indicate fixme's. I plan to address some of these in a separate PR which will come much later given the current workflow. These are existing problems; so the status quo has only improved in this PR imo.

Changelog

changelog = "none"
reason    = "Internal change only."