duckdb: convert aggregate results to duckdb requested output type

[myrrc]

Duckdb widens output aggregation results to avoid overflow. This means
output column may be i128 even if input scalars are not. Our reference_value
doesn't cast duckdb Value to desired type but reinterprets bits which lead
to undefined behaviour.

[codspeed-hq]

Merging this PR will degrade performance by 34.16%

❌ 2 regressed benchmarks
✅ 1899 untouched benchmarks
⏩ 43 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	decompress[u64, (1000, 16)]	74 µs	134.4 µs	-44.94%
❌	Simulation	decode_varbin[(1000, 2)]	61.5 µs	78.1 µs	-21.25%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing myrrc/duckdb-aggregate-output-type (3b63c0a) with develop (3f83ee9)}

Footnotes

1. 43 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩