[DocDB] Add uniqueness validation in the PgGate layer to ensure that hidden tablets are not causing duplicate requests

[rthallamko3]

Jira Link: DB-14048

Description

Per @andrei-mart , PgGate takes the list of tablets as a list of their lower bounds. Each tablet's upper bound is supposed to be next shard's lower_bound. There's no overlaps or gaps possible that way. And the keys must be sorted and unique, lower bound equal or greater than upper bound means empty partition. Apparently PgGate does not verify uniqueness, can cause bugs like #24860 to be introduced.

Add additional debug checks in PgGate layer to honor the uniqueness, so as to prevent issues like #24860 from being introduced again.

Issue Type

kind/enhancement

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