Skip Convert.ChangeType on the same-type object read path

[skuirrels]

What's Changed

NumericVectorDataReader.GetValue(offset, targetType) backs the non-generic read
surface — GetValue(ordinal), this[ordinal], GetValues(object[]). It always
ran the boxed value through Convert.ChangeType, even when targetType already
matched the column's natural type. That is the common case, because those APIs
read using the column's ClrType.

Convert.ChangeType does not early-out on same-type for IConvertible
values: it dispatches through ToXxx() and re-boxes the result, so every value
was boxed twice. This adds a value.GetType() == targetType fast path that
returns the already-boxed value directly.

// after building the boxed `value` from the column
if (value.GetType() == targetType)
{
    return value;
}

if (targetType.IsNumeric())
{
    // Convert.ChangeType (unchanged) …
}

Benchmark

Reading 1,000,000 rows × 3 numeric columns (INTEGER, BIGINT, DOUBLE) via
GetValue(ordinal). BenchmarkDotNet, .NET 10, Apple M4 Pro:

GetValue path	Before	After	Change
Mean time	54.50 ms	22.73 ms	−58%
Allocated	137.3 MB	68.7 MB	−50%

The typed GetFieldValue<T> path is unaffected (~16 ms baseline). The remaining
~69 MB is the unavoidable single box of the object-returning API.

Correctness

Behavior is unchanged: for same-type, ChangeType returned the same value
anyway, and BigInteger (which is not IConvertible) already hit this exact
early-out inside ChangeType. Full existing suite passes locally (6,895 tests).

---

Third in a small series of allocation-focused changes (#336, #337). Source-only;
the benchmark harness is the DuckDB.NET.Benchmarks project from #336.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.59%. Comparing base (6f5a68d) to head (b208fce).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #338   +/-   ##
========================================
  Coverage    87.58%   87.59%           
========================================
  Files           77       77           
  Lines         3134     3136    +2     
  Branches       463      464    +1     
========================================
+ Hits          2745     2747    +2     
  Misses         275      275           
  Partials       114      114           

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[skuirrels]

Just flagging that the macOS red X isn't from this change — it's the Coveralls upload step, which dies trying to brew install its reporter ("Refusing to load formula ... from untrusted tap"). Homebrew changed how it trusts taps and the coveralls action hasn't caught up, so it's failing on macOS for any PR right now, not just this one. The build and tests themselves pass fine on macOS, and Ubuntu/Windows are both green.

Happy to send a small CI fix for it separately if that's helpful.