return mon cors headers on 403

[StekPerepolnen]

Changelog entry

Return CORS headers on 403, mon. Pages show Network Error without CORS; we need CORS so the client can interpret the received response

Changelog category

• Improvement

Description for reviewers

...

[github-actions]

⚪ 2025-07-07 17:24:32 UTC Pre-commit check linux-x86_64-release-asan for b4e106a has started.
⚪ 2025-07-07 17:24:55 UTC Artifacts will be uploaded here
⚪ 2025-07-07 17:28:38 UTC ya make is running...
🟡 2025-07-07 19:04:04 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?
15540	15173	0	137	198	32

🟢 2025-07-07 19:05:27 UTC Build successful.
🟢 2025-07-07 19:05:56 UTC ydbd size 3.9 GiB changed* by -96 Bytes, which is <= 0 Bytes vs main: OK

ydbd size dash	main: fff1a83	merge: b4e106a	diff	diff %
ydbd size	4 200 367 888 Bytes	4 200 367 792 Bytes	-96 Bytes	-0.000%
ydbd stripped size	1 455 387 032 Bytes	1 455 386 968 Bytes	-64 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]

🟢 2025-07-07 18:35:35 UTC The validation of the Pull Request description is successful.

[github-actions]

⚪ 2025-07-07 17:26:13 UTC Pre-commit check linux-x86_64-relwithdebinfo for b4e106a has started.
⚪ 2025-07-07 17:26:27 UTC Artifacts will be uploaded here
⚪ 2025-07-07 17:29:41 UTC ya make is running...
🟡 2025-07-07 18:39:50 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?
37951	35208	0	5	2688	50

⚪ 2025-07-07 18:43:28 UTC ya make is running... (failed tests rerun, try 2)
🟢 2025-07-07 18:54:45 UTC Tests successful.

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

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
421 (only retried tests)	374	0	0	9	38

🟢 2025-07-07 18:54:55 UTC Build successful.
🟢 2025-07-07 18:55:16 UTC ydbd size 2.2 GiB changed* by 0 Bytes, which is <= 0 Bytes vs main: OK

ydbd size dash	main: fff1a83	merge: b4e106a	diff	diff %
ydbd size	2 388 089 616 Bytes	2 388 089 616 Bytes	0 Bytes	0.000%
ydbd stripped size	499 675 880 Bytes	499 675 880 Bytes	0 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}