Consider deferred copying in `string_agg` `GroupsAccumulator`

### Is your feature request related to a problem or challenge?

We currently copy every input string in `update_batch` for `StringAggGroupsAccumulator`.

We could instead just bump the Arc refcount on the input batch and keep `` triples. Then assemble the actual results in `evaluate()` (this is similar to #20504 for `array_agg`). This would be quite a bit more complicated than the current approach, but it could be worth it to reduce the amount of data being copied. It will require some bookkeeping to ensure that the right state is reclaimed after a partial emit.

Note that the current string_agg benchmark uses 3 byte strings, so it would underestimate the impact of this optimization.

### Describe the solution you'd like

_No response_

### Describe alternatives you've considered

_No response_

### Additional context

_No response_

Start at StringAggGroupsAccumulator::update_batch and evaluate(), then compare the deferred-copying approach described for array_agg in #20504. Determine how partial emits reclaim retained batches, and validate that results are assembled correctly while reducing input-string copying.

Written by the indexing model from the issue text.