fix: remove incorrect debug assertion in BatchCoalescer (#9508)

# Which issue does this PR close?

 - Closes #9506


# Rationale for this change

`Vec::reserve(n)` does not guarantee exact capacity, Rust's
`MIN_NON_ZERO_CAP` optimization means `reserve(2)` gives capacity = 4
for most numeric types, causing `debug_assert_eq!(capacity, batch_size)`
to panic in debug mode when `batch_size < 4`.
# What changes are included in this PR?

Replace `reserve` with `reserve_exact` in `ensure_capacity` in both
`InProgressPrimitiveArray` and `InProgressByteViewArray`.
`reserve_exact` skips the amortized growth optimization and allocates
exactly the requested capacity, making the assertion correct.

# Are these changes tested?
No. This only fixes an incorrect debug assertion. 

# Are there any user-facing changes?
No

Author: Tim-53

arrow-select/src/coalesce/byte_view.rs

@@ -101,7 +101,6 @@ impl<B: ByteViewType> InProgressByteViewArray<B> {
         if self.views.capacity() == 0 {
             self.views.reserve(self.batch_size);
         }
-        debug_assert_eq!(self.views.capacity(), self.batch_size);
     }
 
     /// Finishes in progress buffer, if any

arrow-select/src/coalesce/primitive.rs

@@ -58,7 +58,6 @@ impl<T: ArrowPrimitiveType> InProgressPrimitiveArray<T> {
         if self.current.capacity() == 0 {
             self.current.reserve(self.batch_size);
         }
-        debug_assert_eq!(self.current.capacity(), self.batch_size);
     }
 }