fix: Unable to perform joins from a nested object using @link

[Roshan931]

Fixes #1413

Root cause

GraphQL resolver does not descend into schema subtypes, so @link inside a nested object is never resolved

Changes

• When execGraphQLDocument descends into a field with a selection set, it now swaps the active schema to the field's NestedObject fields for object-kind fields (in addition to the existing linked-table swap). This lets linked fields declared inside a subtype be discovered and trigger the linked read request. Added a focused regression test case exercising a mongo @link inside a nested object.

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