fix: preserve duplicate same-timestamp elements in FlinkOrderedListState

[waterWang]

Issue: #39782

Problem: FlinkOrderedListState.readAsMap() builds a SortedMap<Instant, TimestampedValue<T>> using Map.put() keyed by timestamp. When multiple elements share the same timestamp, the second element overwrites the first — causing silent data loss when reading the state.

Fix: Changed to a multimap structure (SortedMap<Instant, List<TimestampedValue<T>>>). Uses Map.computeIfAbsent() to accumulate all values per timestamp bucket, preserving every element. Updated read(), readRange(), and clearRange() to flatten the multimap values.

Root cause (code):

// Before (bug): duplicates lost
SortedMap<Instant, TimestampedValue<T>> sortedMap = Maps.newTreeMap();
for (TimestampedValue<T> value : listValues) {
    sortedMap.put(value.getTimestamp(), value);  // overwrites same-ts elements
}

// After: all values preserved
SortedMap<Instant, List<TimestampedValue<T>>> sortedMap = Maps.newTreeMap();
for (TimestampedValue<T> value : listValues) {
    sortedMap.computeIfAbsent(value.getTimestamp(), k -> new ArrayList<>()).add(value);
}

Testing: FlinkStateInternalsTest — existing ordered-list tests cover ordering semantics; they pass unchanged since the multimap flattens back to the same sorted order. The scoped PR change is behavior-preserving for unique timestamps and fixes duplicate-timestamp data loss.

Docs / changelog: This is a bug fix; no user-facing API change.

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