Pre-size LIST results during materialization

[skuirrels]

Problem

LIST values are materialized into an empty List<T> and grown as each item is added. That creates avoidable backing-array allocations and repeats reflective construction for every row.

Solution

Create normal List<T> results through a reader-local generic factory and pass DuckDB's known list length as the initial capacity. Custom collection targets keep the existing fallback.

Benchmark

100,000 rows with a 16-item List<int>, median of 7 runs after warmup:

Metric	Before	After	Change
Elapsed	95.20 ms	68.58 ms	-28.0%
Allocated	45.02 MiB	14.50 MiB	-67.8%

Tests

• 74 LIST, nested LIST, ARRAY, and type-reader tests passed
• Full suite: 6,897 passed

[codecov]

Codecov Report

❌ Patch coverage is 69.23077% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.53%. Comparing base (fe2abc5) to head (ca3858a).

Files with missing lines	Patch %	Lines
....NET.Data/DataChunk/Reader/ListVectorDataReader.cs	69.23%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #343      +/-   ##
===========================================
- Coverage    87.59%   87.53%   -0.06%     
===========================================
  Files           77       77              
  Lines         3143     3154      +11     
  Branches       466      469       +3     
===========================================
+ Hits          2753     2761       +8     
- Misses         276      278       +2     
- Partials       114      115       +1     

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[Copilot]

Pull request overview

This pull request optimizes LIST/ARRAY materialization in the DuckDB.NET data reader by pre-sizing List<T> instances using DuckDB’s known list length, reducing allocations and repeated reflection overhead during reads.

Changes:

• Adds a regression test asserting List<int> results are created with initial capacity equal to the list length.
• Introduces a cached, reader-local factory path for List<T> to construct new List<T>(capacity) during list materialization.
• Preserves the existing reflection-based fallback for non-List<T> collection targets.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
DuckDB.NET.Test/DuckDBDataReaderListTests.cs	Adds a test verifying list capacity is pre-sized during materialization.
DuckDB.NET.Data/DataChunk/Reader/ListVectorDataReader.cs	Implements cached List<T> factory creation and uses DuckDB-provided length as initial capacity.

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