[Kafka API] fix auth flags check

[a-serebryanskiy]

Changelog entry

Issue: #18598

Original PR to main: #18594

Fixes bug with not forwarding auth token in Kafka proxy in absence of old flag.

Changelog category

• Bugfix

Description for reviewers

...

[github-actions]

⚪ 2025-05-21 08:40:47 UTC Pre-commit check linux-x86_64-release-asan for a1bee10 has started.
⚪ 2025-05-21 08:40:58 UTC Artifacts will be uploaded here
⚪ 2025-05-21 08:43:55 UTC ya make is running...
🟡 2025-05-21 09:55:10 UTC Some tests failed, follow the links below. This fail is not in blocking policy yet Going to retry failed tests...

Test history | Ya make output | Test bloat

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
11844	11640	0	111	59	34

⚪ 2025-05-21 09:56:13 UTC ya make is running... (failed tests rerun, try 2)
🟡 2025-05-21 10:16:34 UTC Some tests failed, follow the links below. This fail is not in blocking policy yet Going to retry failed tests...

Test history | Ya make output | Test bloat | Test bloat

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
248 (only retried tests)	167	0	7	43	31

⚪ 2025-05-21 10:16:43 UTC ya make is running... (failed tests rerun, try 3)
🟡 2025-05-21 10:36:48 UTC Some tests failed, follow the links below. This fail is not in blocking policy yet

Test history | Ya make output | Test bloat | Test bloat | Test bloat

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
107 (only retried tests)	38	0	7	31	31

🟢 2025-05-21 10:36:56 UTC Build successful.
🟢 2025-05-21 10:37:26 UTC ydbd size 3.6 GiB changed* by +784 Bytes, which is < 100.0 KiB vs stable-25-1: OK

ydbd size dash	stable-25-1: 1468e22	merge: a1bee10	diff	diff %
ydbd size	3 912 883 368 Bytes	3 912 884 152 Bytes	+784 Bytes	+0.000%
ydbd stripped size	1 368 554 128 Bytes	1 368 554 384 Bytes	+256 Bytes	+0.000%

^{*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]

⚪ 2025-05-21 08:41:52 UTC Pre-commit check linux-x86_64-relwithdebinfo for a1bee10 has started.
⚪ 2025-05-21 08:42:03 UTC Artifacts will be uploaded here
⚪ 2025-05-21 08:44:51 UTC ya make is running...
🟡 2025-05-21 09:40:31 UTC Some tests failed, follow the links below. Going to retry failed tests...

Test history | Ya make output | Test bloat

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
18677	17400	0	5	1212	60

⚪ 2025-05-21 09:42:06 UTC ya make is running... (failed tests rerun, try 2)
🟢 2025-05-21 10:02:25 UTC Tests successful.

Test history | Ya make output | Test bloat | Test bloat

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
128 (only retried tests)	66	0	0	5	57

🟢 2025-05-21 10:02:32 UTC Build successful.
🟢 2025-05-21 10:02:51 UTC ydbd size 2.1 GiB changed* by +544 Bytes, which is < 100.0 KiB vs stable-25-1: OK

ydbd size dash	stable-25-1: 1468e22	merge: a1bee10	diff	diff %
ydbd size	2 249 300 760 Bytes	2 249 301 304 Bytes	+544 Bytes	+0.000%
ydbd stripped size	476 023 928 Bytes	476 024 056 Bytes	+128 Bytes	+0.000%

^{*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]

🟢 2025-05-21 08:44:03 UTC The validation of the Pull Request description is successful.