feat(physical-plan): expose finalized Accumulator state on BoundedWindowAggExec

[avantgardnerio]

Summary

Adds a getter on BoundedWindowAggExec that returns the finalized Accumulator::state() for each aggregate window expression at partition close, keyed by PARTITION BY tuple. This lets callers observe per-partition final aggregate state without emitting it as an output column on every row.

The change is purely additive:

• New public type alias FinalizedPartitionState = HashMap<PartitionKey, Vec<Option<Vec<ScalarValue>>>> — one Accumulator::state() per (PARTITION BY tuple, window expression) pair. Non-aggregate window functions (row_number, rank, lead/lag, ...) occupy None at their slot in the inner Vec.
• New field finalized_state: Arc<[Mutex<FinalizedPartitionState>]> on BoundedWindowAggExec — one slot per output partition.
• New public method BoundedWindowAggExec::finalized_partition_state(partition) — returns the current snapshot for a given output partition.
• Write site at the top of prune_state in BoundedWindowAggStream — snapshots every entry whose state.is_end is set, immediately before either prune_out_columns or prune_partition_batches drops it. This covers both mid-stream partition close (as SQL PARTITION BY groups end) and end-of-stream with a single write path.

No trait changes: the existing public Accumulator::state() method (already used by the group-by Partial/Final protocol) is the state source. Behavior without a reader is unchanged — the slots simply hold state that no one reads.

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

❌ Patch coverage is 80.74074% with 26 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.85%. Comparing base (62cfc0c) to head (61c2aa8).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
...ysical-plan/src/windows/bounded_window_agg_exec.rs	80.74%	11 Missing and 15 partials ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #24007    +/-   ##
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  Coverage   80.85%   80.85%            
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  Files        1096     1099     +3     
  Lines      373911   374473   +562     
  Branches   373911   374473   +562     
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+ Hits       302329   302798   +469     
- Misses      53547    53611    +64     
- Partials    18035    18064    +29     

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