fix(rpc): move submit proof checks off async runtime

[Ollie202]

Partially addresses #1976.

Problem

The RPC transaction submission endpoints perform CPU-heavy proof work directly inside async request handlers:

• SubmitProvenTx verifies the submitted transaction proof with TransactionVerifier::verify.
• SubmitProvenTxBatch locally re-proves the proposed batch to compare it with the submitted batch proof.

Both operations can block Tokio worker threads while the RPC service is handling requests.

Changes

• Move SubmitProvenTx transaction proof verification into spawn_blocking_in_current_span.
• Move SubmitProvenTxBatch local batch proof comparison into spawn_blocking_in_current_span.
• Preserve the existing invalid-argument errors for proof failures.
• Return internal errors only if the blocking task itself fails to join.
• Add a changelog entry for the follow-up Execution and signing is blocking in an async runtime #1976 fix.

Testing

• cargo +nightly fmt --check --all
• git diff --check

I also tried cargo check -p miden-node-rpc and cargo check --manifest-path crates/rpc/Cargo.toml, but both stop during dependency checkout on Windows because the current protocol dependency pulls an AggLayer submodule file with : in the filename:

tools/addRollupType/addRollupMainnet10/add_rollup_type_10_output-2025-03-11T15:54:21.263Z.json