Properly handle index build result in KQP scheme query

[UgnineSirdis]

Changelog entry

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Changelog category

• Not for changelog (changelog entry is not required)

Description for reviewers

Scheme queries returned success even if index had not been built. I fixed this issue. The corresponding tests can be made (and are already made) in the next PR #20133 for unsuccessful unique index build.

[github-actions]

🟢 2025-06-26 20:44:07 UTC The validation of the Pull Request description is successful.

[github-actions]

⚪ 2025-06-26 20:38:42 UTC Pre-commit check linux-x86_64-relwithdebinfo for 1ea54c9 has started.
⚪ 2025-06-26 20:38:53 UTC Artifacts will be uploaded here
⚪ 2025-06-26 20:42:08 UTC ya make is running...
🟡 2025-06-26 21:43:05 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?
29176	27777	0	5	1364	30

⚪ 2025-06-26 21:45:30 UTC ya make is running... (failed tests rerun, try 2)
🟢 2025-06-26 21:55:24 UTC Tests successful.

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

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
143 (only retried tests)	117	0	0	2	24

🟢 2025-06-26 21:55:31 UTC Build successful.
🟢 2025-06-26 21:55:50 UTC ydbd size 2.2 GiB changed* by +5.2 KiB, which is < 100.0 KiB vs main: OK

ydbd size dash	main: 935b735	merge: 1ea54c9	diff	diff %
ydbd size	2 383 680 216 Bytes	2 383 685 576 Bytes	+5.2 KiB	+0.000%
ydbd stripped size	499 053 288 Bytes	499 054 312 Bytes	+1.0 KiB	+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]

⚪ 2025-06-26 20:38:49 UTC Pre-commit check linux-x86_64-release-asan for 1ea54c9 has started.
⚪ 2025-06-26 20:39:01 UTC Artifacts will be uploaded here
⚪ 2025-06-26 20:42:19 UTC ya make is running...
🟡 2025-06-26 22:26:22 UTC Some tests failed, follow the links below. This fail is not in blocking policy yet

Test history | Ya make output | Test bloat

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
14043	13655	0	94	272	22

🟢 2025-06-26 22:27:36 UTC Build successful.
🟢 2025-06-26 22:28:03 UTC ydbd size 3.9 GiB changed* by +9.7 KiB, which is < 100.0 KiB vs main: OK

ydbd size dash	main: 9a3ba4f	merge: 1ea54c9	diff	diff %
ydbd size	4 193 756 216 Bytes	4 193 766 176 Bytes	+9.7 KiB	+0.000%
ydbd stripped size	1 453 748 792 Bytes	1 453 751 288 Bytes	+2.4 KiB	+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}