starknet_os_flow_tests,starknet_os,starknet_transaction_prover: test the Cairo digest via verifier

[einat-starkware]

Part 5 of the single-proof verification stack; last PR of milestone 1. Extracts the Cairo digest entry-point runner (starknet_os::test_utils::proof_fact_fold_runner) and the verifier-task fixtures (starknet_transaction_prover::verifier_task::test_utils) into shared test utilities, and adds the combined test:

• test_cairo_processed_proof_digest_matches_circuit_verifier_output: the Cairo0 processed-proof output digest over the fixture's proof facts satisfies the real circuit verifier's output comparison.

The OS-in-the-loop test (test_os_emitted_output_feeds_verifier_task_comparison) moved to #15064, since it needs the digest in the OS output.

Stack (restructured):

• Milestone 1: apollo_starknet_os_program,starknet_os: add the Cairo0 processed-proof digests #15092 ← apollo_starknet_os_program,starknet_os: cite the circuit hashes' source and add todos #15086 ← starknet_transaction_prover,workspace: run the circuit verifier as a bootloader task #15088 ← this PR
• Milestone 2, the digest in the OS output: ← apollo_starknet_os_program,starknet_os,blockifier: emit the single-proof digest in the OS output #15064

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https://claude.ai/code/session_01E43QxCycT6DpDJo435Ft8R

[cursor]

PR Summary

Low Risk
Test-only refactors and a new integration test; production verifier/OS paths are unchanged aside from exposing test_utils under the testing feature.

Overview
Extracts shared test helpers for proof-fact Cairo digest checks and circuit-verifier fixtures, then adds an end-to-end flow test that ties them together.

Cairo0 digest assertions move from proof_fact_fold_test.rs into starknet_os::test_utils::proof_fact_fold_runner, and verifier gzip fixtures / run_one_leaf_verifier_task move into starknet_transaction_prover::verifier_task::test_utils behind a new testing feature (optional flate2). starknet_os’s testing feature also pulls in apollo_starknet_os_program/test_programs.

test_cairo_processed_proof_digest_matches_circuit_verifier_output in starknet_os_flow_tests runs the real one-leaf circuit verifier on the shared fixture, checks leaf/combined digests against Cairo entry points, and asserts verify_circuit_verifier_task_output accepts the processed-proof output digest.

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[reviewable-StarkWare]

This change is 

[einat-starkware]

Closing: the stack now runs the circuit verifier in Cairo, from the aggregator (#15161, #15163, #15162), instead of as a Rust bootloader task, so this test of the Rust task is no longer needed.

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