Add a deterministic unit test that exercises the writeConflictRetry block at src/mongo/db/index_builds/multi_index_block.cpp:1240, which wraps IndexBuildInterceptor::recordDuplicateKey during the bulk-load commit of a unique index

1. Build a unique index on a collection that contains at least one true duplicate so the recordDuplicateKey
     callback is invoked
2. Drive the build through the load phase up to (and including) _indexes[i].bulk->commit.
3. Immediately before the call that will trigger duplicate recording, arm the engine-agnostic write-conflict failpoint 

1. After commit returns, assert all three:
   1. The build commit succeeded (the retry did its job).
   2. Exactly one WCE fired
   3. The duplicate key was recorded exactly once in the duplicate-key tracking store the interceptor owns. (Count records directly in the underlying ident; do not just check "the read returned something" – a buggy retry  could insert the duplicate twice and a weak read assertion would pass)

  Suggested test name: DuplicateKeyRecordingSurvivesWriteConflict.

  src/mongo/db/index_builds/multi_index_block_test.cpp.

  If exercising the duplicate path from MultiIndexBlockTest proves hard (e.g. the test fixture can't easily
  run a unique build with intentional dupes through insertAllDocumentsInCollection), route through
  IndexBuildsManagerTest (see src/mongo/db/index_builds/index_builds_manager_test.cpp) –

Constraints

  * Use the engine-agnostic helper enableStorageEngineWriteConflictForWrites introduced by SERVER-126328, not the WT-specific WTWriteConflictException failpoint by name.
  * FailPoint::Mode::nTimes only – no Mode::random. Per the SPM-4469 strategic split, percentage-based
  WCE injection is jstest-level (SERVER-126326) only.
  * Test-only PR – do not modify production code.
  * Depends on SERVER-126385 landing first (it brings in enableStorageEngineWriteConflictForWrites +
  CatalogTestFixture plumbing).

Acceptance

• New DuplicateKeyRecordingSurvivesWriteConflict test passes.
• All existing tests in +multi_index_block_test (or +index_builds_manager_test if routed there) pass.
  WCE-fired-exactly-once assertion present.
• Exactly-one-duplicate-record-persisted assertion present (record-count check against the duplicate-key ident,  not just a read).
  No production code changed.
• No Mode::random, no jstest changes.