fix(store): move local block proving off async runtime

[Ollie202]

Partially addresses #1976.

Problem

BlockProver::prove is async, but the local store prover path calls LocalBlockProver::prove directly. Local block proving is CPU-heavy synchronous work, so running it on a Tokio worker can starve the async runtime.

Changes

• Move local store block proving into spawn_blocking_in_current_span.
• Clone the block header into the blocking task so the prover call owns everything it needs.
• Add a separate StoreProverError variant for blocking task join failures.
• Add a changelog entry for the follow-up fix.

Testing

• rustfmt crates/store/src/server/block_prover_client.rs
• git diff --check

I also tried cargo check -p miden-node-store and cargo check --manifest-path crates/store/Cargo.toml, but both stop before compiling because the current protocol git dependency tries to check out an agglayer submodule file with : in the filename on Windows:

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