Investigate remaining MutableSegmentImpl FIXMEs around upsert consistency and aggregateMetrics

### Motivation
`pinot-segment-local/src/main/java/org/apache/pinot/segment/local/indexsegment/mutable/MutableSegmentImpl.java` still has two FIXME blocks:
- around line 187: segment load during consumption can update record location without updating valid doc IDs for a newer late event
- around line 1483: `getOrCreateDocId()` still warns that aggregateMetrics breaks for multi-value dimensions and references closed issue #3867

These comments point at correctness-sensitive realtime behavior and should either be fixed or reconciled with current behavior and tests.

### Scope
- Reproduce or rule out the valid-doc-id inconsistency during concurrent consumption plus segment load.
- Re-evaluate aggregateMetrics behavior for MV dimensions and decide whether code, tests, or comments are stale.
- Add regression coverage for any confirmed bug.
- Remove or update the FIXMEs once behavior is understood.

### Notes
Observed on `upstream/master` on April 15, 2026.

Start in pinot-segment-local/src/main/java/org/apache/pinot/segment/local/indexsegment/mutable/MutableSegmentImpl.java at the FIXME blocks around lines 187 and 1483. Reproduce or rule out the concurrent segment-load inconsistency, then inspect current aggregateMetrics behavior for multi-value dimensions and related tests. Done means confirmed behavior has regression coverage where needed and both FIXMEs are removed or updated.

Written by the indexing model from the issue text.