blockifier_test_utils: fix ETXTBSY race in cairo package download

[avi-starkware]

verify_cairo1_package double-checks cairo1_package_exists — once
without the lock (fast path) and once after acquiring it. The predicate
only tested .exists() on the two binaries, so a concurrent process
could observe the package as "ready" while tar was still mid-write:

1. Process A holds the download lock, tar is extracting the archive.
2. tar extracts sequentially: first binary is closed, second binary
   is created (so .exists() is true) and being written to.
3. Process B calls cairo1_package_exists, sees both files, skips the
   lock.
4. Process B forks + execs the second binary → Linux returns
   ETXTBSY because the file is still open for writing by tar.

Fix with the same commit-marker pattern used in compile_cache.rs:
write .download_complete after tar finishes, and require it in
cairo1_package_exists. Until the marker exists, concurrent callers
fall through to the lock and wait for the writer.

Co-Authored-By: Claude Opus 4.7 (1M context) noreply@anthropic.com

[reviewable-StarkWare]

This change is 

[avi-starkware]

• blockifier_test_utils: fix ETXTBSY race in cairo package download #13839 👈 (View in Graphite)
• main-v0.14.2

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PR Summary

Low Risk
Low risk: small change in test utility download logic that adds an extraction completion marker to avoid concurrent processes executing half-written binaries.

Overview
Prevents a race where concurrent verify_cairo1_package callers could observe the Cairo1 compiler binaries as present while tar was still writing them, leading to ETXTBSY on exec.

This writes a .download_complete marker at the end of download_cairo_package and updates cairo1_package_exists to require that marker in addition to the two compiler binaries.

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[dorimedini-starkware]

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