Antalya-26.6 - Fix races in the export_merge_tree_part test family

[CarlosFelipeOR]

Changelog category (leave one):

• Build/Testing/Packaging Improvement

CI/CD Options

Exclude tests:

• Fast test
• Integration Tests
• Stateless tests
• Stateful tests
• Performance tests
• Aarch64 tests
• All with ASAN
• All with TSAN
• All with MSAN
• All with UBSAN
• All with Coverage
• All Regression
• Disable CI Cache

Regression jobs to run:

• Fast suites (mostly <1h)
• Aggregate Functions (2h)
• Alter (1.5h)
• Benchmark (30m)
• CAS (content-addressed storage; Antalya only)
• ClickHouse Keeper (1h)
• Iceberg (2h)
• LDAP (1h)
• OAuth (5m)
• Parquet (1.5h)
• RBAC (1.5h)
• SSL Server (1h)
• S3 (2h)
• S3 Export (2h)
• Swarms (30m)
• Tiered Storage (2h)

[github-actions]

Workflow [PR], commit [ff4c649]

[arthurpassos]

I swear I thought I had already implemented this. It either got lost during ports or I abandoned this for a reason

[CarlosFelipeOR]

@arthurpassos , I’ve addressed all your comments. Could you please review it again?

[arthurpassos]

LGTM, assuming CICD is happy with it

[arthurpassos]

Nit: I think it's better if you provide the partition id or transaction id you are waiting for instead of the count.

In any case, I suppose this is good enough for testing code

[CarlosFelipeOR]

QA Verification — ✅

All four tests changed in this PR passed on every job.

The remaining failures are pre-existing and unrelated to this PR:

• 03572_export_merge_tree_part_limits_and_table_functions — not changed here. The same -5 / +4 file-count diff on big_destination_max_bytes already appears on Antalya-26.6: Datalake catalog auth token profile events #2222, Parquet v3 read concurrency #2235, [experimental] CAS (Content-addressed storage) over shared object storage for antalya-26.6 #2159 and on branch runs with no PR. The randomized-settings diagnosis re-ran it 6× with the same settings: 6/6 passed.
• Stress test (amd_debug, amd_tsan, arm_tsan) — Cannot start clickhouse-server, Check failed, Unknown error; recurring across 13–14 distinct PRs since July.