Overview

Add unit test coverage for PDIB side-write draining into a unique index with TrackDuplicates::kTrack, verifying that duplicate keys are
recorded to the constraint violations table and that the additional container oplog entry is generated.

Background

The existing PDIB drain tests added in SERVER-114680 all use non-unique indexes with TrackDuplicates::kNoTrack and dupsAllowed=true.
There is a separate code path exercised when draining into a unique index with kTrack: duplicate keys hit the onDuplicateKeyFn callback
(index_build_interceptor.cpp:153-157), which calls recordDuplicateKey(), which under PDIB uses container_write::insert() to write to
the constraint violations table (duplicate_key_tracker.cpp:97-122). This generates an additional CI oplog entry beyond the 2 (CI for index
insert, CD for side write removal) that the existing tests verify. No existing test exercises this end-to-end path through drain.

Scope of Work

• src/mongo/db/index_builds/index_build_interceptor_test.cpp — Add a new test (e.g.
  DrainInsertSideWriteRecordsDuplicateKeyAndGeneratesContainerOplogEntriesPrimaryDriven)

Implementation Details

The new test should:

1. Enable PDIB feature flags (featureFlagContainerWrites, featureFlagPrimaryDrivenIndexBuilds).
2. Register the OpObserver (call registerOpObserver()).
3. Create an IndexBuildInfo with a unique index spec
4. Pre-populate the index with a key (e.g. key value 10, RecordId 1) using insertKeys() directly.
5. Buffer a side write that inserts a duplicate of that key (same key value 10, different RecordId 2) via {{interceptor->sideWrite(...,
   Op::kInsert, ...)}}.
6. Record oplog size after buffer phase.
7. Call interceptor->drainWritesIntoIndex() with InsertDeleteOptions{.dupsAllowed = true}, TrackDuplicates::kTrack,
   DrainYieldPolicy::kNoYield.
8. Verify:
   • The oplog contains an applyOps entry with 3 inner container operations (not 2):
     • CI on the index ident (the duplicate key insert)
     • CI on the constraint violations table ident (the recorded duplicate)
     • CD on the side writes table ident (the consumed side write)
   • The constraint violations table is non-empty (use getDuplicateKeyTableContents()).
   • The side writes table is empty (use getSideWritesTableContents()).

Follow the structure of DrainInsertSideWriteGeneratesContainerOplogEntriesPrimaryDriven (line 844) as a template, adapting it for a unique
index with a pre-existing key conflict and kTrack.

Acceptance Criteria

• New test passes: bazel run +index_build_interceptor_test
• Test verifies 3 container ops in the drain oplog entry (CI index, CI constraint violations, CD side writes)
• Test verifies constraint violations table contains the recorded duplicate
• Test verifies side writes table is empty after drain