[codex] Expose typed generate-pie errors

[prkpndy]

Summary

• re-export StateUpdateError through generate_pie::error so downstream callers can match it structurally
• preserve BlockProcessingError as the typed source inside PieGenerationError::BlockProcessing
• add an integration test that exercises the public error API from outside the crate

Why

This is a follow-up to #525.

The previous change introduced structured StateUpdateError and BlockProcessingError variants, but two API boundaries still erased that structure for downstream users:

• StateUpdateError lived in a private module, so external callers could not match StateUpdateError::RpcError
• PieGenerationError::BlockProcessing stored its source as Box<dyn Error + Send + Sync>, which forced downstream callers to downcast instead of pattern matching directly

This follow-up makes the typed error model usable to consumers such as the orchestrator.

Validation

• cargo fmt --all
• cargo test -p generate-pie
• cargo test -p generate-pie --test public_error_api
• source /Users/prakhar/work/karnot/snos/sequencer_venv/bin/activate && cargo clippy -p generate-pie --all-features --tests --no-deps -- -D warnings