refactor: stream spill triggering for partial aggregation

[dqhl76]

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Summary

WIP

part of #18906

Replace per-block “spill immediately” with a batch/stream policy so spilled fragments are reasonably sized. This reduces tiny files and I/O amplification.

uv run compare_query_stats.py 002bb805-0dd8-4bdc-b181-bdda52ff39c2 45450639-3cde-48cf-a519-c617c5dbc34e AggregateFinal
Comparing node 'AggregateFinal':
  Query A: 002bb805-0dd8-4bdc-b181-bdda52ff39c2
  Query B: 45450639-3cde-48cf-a519-c617c5dbc34e
-------------------------------------------------------------------------------
  Metric                Unit                Query A       Query B       Δ (B-A)
-------------------------------------------------------------------------------
  RemoteSpillReadCount  Count                 4,472            86        -4,386
  RemoteSpillReadBytes  Bytes            11,511,760       172,033   -11,339,727
  RemoteSpillReadTime   MillisSeconds         4,012            15        -3,997
-------------------------------------------------------------------------------
uv run compare_query_stats.py 002bb805-0dd8-4bdc-b181-bdda52ff39c2 45450639-3cde-48cf-a519-c617c5dbc34e AggregatePartial
Comparing node 'AggregatePartial':
  Query A: 002bb805-0dd8-4bdc-b181-bdda52ff39c2
  Query B: 45450639-3cde-48cf-a519-c617c5dbc34e
-----------------------------------------------------------------------------------
  Metric                 Unit                 Query A        Query B        Δ (B-A)
-----------------------------------------------------------------------------------
  RemoteSpillWriteCount  Count                     52             86            +34
  RemoteSpillWriteBytes  Bytes             11,511,760         90,575    -11,421,185
  RemoteSpillWriteTime   MillisSeconds            355             44           -311
-----------------------------------------------------------------------------------

Tests

• Unit Test
• Logic Test
• Benchmark Test
• No Test - Explain why

Type of change

• Bug Fix (non-breaking change which fixes an issue)
• New Feature (non-breaking change which adds functionality)
• Breaking Change (fix or feature that could cause existing functionality not to work as expected)
• Documentation Update
• Refactoring
• Performance Improvement
• Other (please describe):

---

This change is 

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[github-actions]

Docker Image for PR

• tag: pr-18943-9447b3a-1762948324

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Docker Image for PR

• tag: pr-18943-a064856-1763005916

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ClickBench Report

• hits: https://benchmark.databend.com/clickbench/pr/18943/19319151039/hits.html
• tpch: https://benchmark.databend.com/clickbench/pr/18943/19319151039/tpch.html

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[github-actions]

Docker Image for PR

• tag: pr-18943-0667c1f-1763088331

note: this image tag is only available for internal use.

[github-actions]

ClickBench Report

• hits: https://benchmark.databend.com/clickbench/pr/18943/19351665467/hits.html
• tpch: https://benchmark.databend.com/clickbench/pr/18943/19351665467/tpch.html