25-1-3: schemeshard: quick-fix tenant system tablets deletion

[ijon]

Merge from main:

• a721d56 -- schemeshard: quick-fix, delete tenant system tablet directly in root hive #21377
• 1f8359c -- schemeshard: fix SyncTenant handling for dropped extsubdomains #22360

Currently, drop-extsubdomain operations may not completely remove dedicated databases from the system:

• system tablets of the tenant may remain in the root hive
• traces of database paths and subdomain objects may persist in the root schemeshard

Root cause: The cleanup procedure depends on the tenant hive's existence - delete-tablet requests are sent to the tenant hive for redirection to the root hive. This works for most databases but fails with dedicated databases, where the tenant hive cannot outlive drop-extsubdomain operations, and there's no guarantee that tenant nodes and tablets will be available during deletion.

Changes:

• Implement quick fix for subdomain cleanup by directly sending system tablets delete-tablet requests to the root hive (only for dedicated databases)
• Add unit and reboot tests to verify proper extsubdomain cleanup
• Fix aggressive reconnect loop in coordinator's mediator_queue
• Fix handling of TEvSchemeShard::TEvSyncTenantSchemeShard in schemeshard

Closes #19842.

Changelog entry

Fix issue where dedicated database deletion may leave database system tablets improperly cleaned.

Changelog category

• Bugfix

[github-actions]

⚪ 2025-08-06 15:18:42 UTC Pre-commit check linux-x86_64-release-asan for 3ca24e1 has started.
⚫ 2025-08-06 15:19:43 UTC Check cancelled

[github-actions]

🟢 2025-08-06 15:19:11 UTC The validation of the Pull Request description is successful.

[github-actions]

⚪ 2025-08-07 11:13:07 UTC Pre-commit check linux-x86_64-release-asan for dc311dc has started.
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⚪ 2025-08-07 11:17:12 UTC ya make is running...
🟡 2025-08-07 12:27:52 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?
12683	12486	0	53	110	34

⚪ 2025-08-07 12:29:04 UTC ya make is running... (failed tests rerun, try 2)
🟡 2025-08-07 12:55:58 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?
254 (only retried tests)	197	0	6	16	35

⚪ 2025-08-07 12:56:11 UTC ya make is running... (failed tests rerun, try 3)
🟡 2025-08-07 13:18:01 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?
86 (only retried tests)	41	0	4	7	34

🟢 2025-08-07 13:18:10 UTC Build successful.
🟡 2025-08-07 13:18:42 UTC ydbd size 3.7 GiB changed* by +119.6 KiB, which is >= 100.0 KiB vs stable-25-1-3: Warning

ydbd size dash	stable-25-1-3: 212c3eb	merge: dc311dc	diff	diff %
ydbd size	4 021 544 832 Bytes	4 021 667 312 Bytes	+119.6 KiB	+0.003%
ydbd stripped size	1 402 629 432 Bytes	1 402 672 728 Bytes	+42.3 KiB	+0.003%

^{*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]

⚪ 2025-08-07 11:13:15 UTC Pre-commit check linux-x86_64-relwithdebinfo for dc311dc has started.
⚪ 2025-08-07 11:13:29 UTC Artifacts will be uploaded here
⚪ 2025-08-07 11:16:27 UTC ya make is running...
🟡 2025-08-07 12:23:01 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?
19826	18534	0	3	1243	46

⚪ 2025-08-07 12:55:04 UTC ya make is running... (failed tests rerun, try 2)
🟢 2025-08-07 13:16:59 UTC Tests successful.

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

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
104 (only retried tests)	60	0	0	4	40

🟢 2025-08-07 13:17:07 UTC Build successful.
🟢 2025-08-07 13:17:29 UTC ydbd size 2.2 GiB changed* by +55.8 KiB, which is < 100.0 KiB vs stable-25-1-3: OK

ydbd size dash	stable-25-1-3: 212c3eb	merge: dc311dc	diff	diff %
ydbd size	2 310 337 024 Bytes	2 310 394 152 Bytes	+55.8 KiB	+0.002%
ydbd stripped size	485 378 656 Bytes	485 392 672 Bytes	+13.7 KiB	+0.003%

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