test(tsd): assert validationErrors type error in bulkWrite #15768

[THE-Amrit-mahto-05]

Correct tsd tests for conn.bulkWrite so that accessing validationErrors on
the result is properly flagged as a type error by TypeScript. Previously,
TypeScript did not catch this without a cast, which could mislead developers
into thinking validationErrors exists directly on ClientBulkWriteResult.

Problem

TSD tests did not fail when trying to access `res3.validationErrors` directly.
This could cause developers to assume the property exists on the bulkWrite result,
which is incorrect.

Changes

- Added `expectError((res3 as typeof res3 & { validationErrors?: unknown }).validationErrors);`
  to properly assert the type error.
- Updated the test file `test/types/connection.test.ts`.

Expected Behavior

TypeScript now fails when trying to access `validationErrors` directly on
`ClientBulkWriteResult`. The property only exists under `res3.mongoose?.validationErrors`.