[Kafka API] fix auth flags check

[a-serebryanskiy]

Changelog entry

Issue: #18598

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 08:14:47 UTC Pre-commit check linux-x86_64-relwithdebinfo for dbb9798 has started.
⚪ 2025-05-21 08:15:01 UTC Artifacts will be uploaded here
⚪ 2025-05-21 08:18:00 UTC ya make is running...
🟡 2025-05-21 09:33:00 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?
20591	19212	0	6	1330	43

⚪ 2025-05-21 09:34:44 UTC ya make is running... (failed tests rerun, try 2)
🟢 2025-05-21 09:59:49 UTC Tests successful.

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

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
328 (only retried tests)	287	0	0	4	37

🟢 2025-05-21 09:59:57 UTC Build successful.
🟢 2025-05-21 10:00:17 UTC ydbd size 2.2 GiB changed* by +528 Bytes, which is < 100.0 KiB vs main: OK

ydbd size dash	main: d0e7a11	merge: dbb9798	diff	diff %
ydbd size	2 355 579 680 Bytes	2 355 580 208 Bytes	+528 Bytes	+0.000%
ydbd stripped size	494 877 352 Bytes	494 877 480 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 08:37:09 UTC The validation of the Pull Request description is successful.

[github-actions]

⚪ 2025-05-21 08:16:39 UTC Pre-commit check linux-x86_64-release-asan for dbb9798 has started.
⚪ 2025-05-21 08:16:50 UTC Artifacts will be uploaded here
⚪ 2025-05-21 08:19:42 UTC ya make is running...
🟡 2025-05-21 09:48:08 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?
12891	12725	0	105	36	25

⚪ 2025-05-21 09:49:18 UTC ya make is running... (failed tests rerun, try 2)
🟡 2025-05-21 10:22:35 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?
1506 (only retried tests)	1399	0	62	21	24

⚪ 2025-05-21 10:22:53 UTC ya make is running... (failed tests rerun, try 3)
🟡 2025-05-21 10:51:33 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?
1097 (only retried tests)	984	0	63	27	23

🟢 2025-05-21 10:51:47 UTC Build successful.
🟢 2025-05-21 10:52:22 UTC ydbd size 3.9 GiB changed* by +848 Bytes, which is < 100.0 KiB vs main: OK

ydbd size dash	main: d0e7a11	merge: dbb9798	diff	diff %
ydbd size	4 143 779 888 Bytes	4 143 780 736 Bytes	+848 Bytes	+0.000%
ydbd stripped size	1 438 160 664 Bytes	1 438 160 984 Bytes	+320 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}