Maps: Avoid double iteration in uniqueIndex for FilteredCollection

[nickita-khylkouski]

Summary

• Skip pre-sizing optimization in Maps.uniqueIndex when input is FilteredCollection or FilteredMultimapValues
• Prevents double iteration since these classes have O(n) size() methods

Motivation

Maps.uniqueIndex calls size() on Collection inputs to pre-size the builder. However, FilteredCollection.size() and FilteredMultimapValues.size() iterate through all elements applying the predicate. This causes the predicate to be evaluated twice:

1. Once during size() call
2. Once during iteration to build the map

This fix skips the pre-sizing optimization for these filtered views, avoiding the double iteration.

Fixes #6638

Testing

• Standalone test verified predicate is called once per element (4 calls), not twice (8 calls)
• Basic functionality verified with normal collections