fix data race in the ydb workload topic run: do not initialize the test mock on the production code path

[ubyte]

Changelog entry

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Changelog category

• Not for changelog (changelog entry is not required)

Description for reviewers

LOGBROKER-9919

[github-actions]

🟢 2025-10-10 17:59:31 UTC The validation of the Pull Request description is successful.

[github-actions]

⚪ 2025-10-10 15:16:10 UTC Pre-commit check linux-x86_64-release-asan for 7fae9c9 has started.
⚪ 2025-10-10 15:16:23 UTC Artifacts will be uploaded here
⚪ 2025-10-10 15:19:00 UTC ya make is running...
🟡 2025-10-10 16:11:51 UTC Some tests failed, follow the links below. This fail is not in blocking policy yet

Ya make output | Test bloat

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
1608	1429	0	30	146	3

🟢 2025-10-10 16:11:54 UTC Build successful.
🟡 2025-10-10 16:12:10 UTC ydbd size 3.7 GiB changed* by +937.7 KiB, which is >= 100.0 KiB vs main: Warning

ydbd size dash	main: a446a72	merge: 7fae9c9	diff	diff %
ydbd size	4 023 586 624 Bytes	4 024 546 800 Bytes	+937.7 KiB	+0.024%
ydbd stripped size	1 494 478 880 Bytes	1 494 903 392 Bytes	+414.6 KiB	+0.028%

^{*please be aware that the difference is based on comparing your commit and the last completed build from the post-commit, check comparation}

[github-actions]

⚪ 2025-10-10 15:16:16 UTC Pre-commit check linux-x86_64-relwithdebinfo for 7fae9c9 has started.
⚪ 2025-10-10 15:16:36 UTC Artifacts will be uploaded here
⚪ 2025-10-10 15:19:51 UTC ya make is running...
🟢 2025-10-10 15:53:09 UTC Tests successful.

Ya make output | Test bloat

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
2489	2478	0	0	6	5

🟢 2025-10-10 15:53:14 UTC Build successful.
🟡 2025-10-10 15:53:26 UTC ydbd size 2.3 GiB changed* by +609.3 KiB, which is >= 100.0 KiB vs main: Warning

ydbd size dash	main: a446a72	merge: 7fae9c9	diff	diff %
ydbd size	2 418 700 232 Bytes	2 419 324 152 Bytes	+609.3 KiB	+0.026%
ydbd stripped size	515 107 272 Bytes	515 299 176 Bytes	+187.4 KiB	+0.037%

^{*please be aware that the difference is based on comparing your commit and the last completed build from the post-commit, check comparation}