feat: add inner_product scalar function

[crm26]

Which issue does this PR close?

Part of #21536 — split of #21371 into one-function-per-PR.

Rationale for this change

Adds inner_product(array1, array2) — the dot product of two equal-length numeric arrays, returning Float64. Computed as sum(array1[i] * array2[i]).

What changes are included in this PR?

Mirrors the structural pattern of merged #21542 (cosine_distance):

• Same coerce_types for List/LargeList/FixedSizeList of any numeric inner type, with widening to LargeList when any input is LargeList (per the fix: array_concat widens container variant for mixed List/LargeList inputs #21704 pattern)
• Same NULL semantics: bare NULL → NULL, NULL row → NULL, NULL element in list → NULL
• Same Arrow-idiomatic implementation: single as_float64_array(list_array.values()) downcast, slice by value_offsets(), iterate via ScalarBuffer<f64>
• No alias, no shared module — standalone, inline math

The arithmetic is the only semantic divergence from cosine_distance:

• dot += a*b (no magnitude or normalization)
• Empty arrays return 0.0 (sum of empty set), not NULL
• No zero-magnitude special case (inner_product([0,0], [1,2]) returns 0, which is well-defined for inner product)

Are these changes tested?

Yes. SLT covers:

• Orthogonal, identical, opposite, general non-trivial vectors
• Single zero vector, both zero vectors
• Bare NULL in either or both positions
• NULL element inside a list (returns NULL for that row)
• Mismatched lengths (error)
• LargeList inputs
• Mixed (List, LargeList) in both orders
• (FixedSizeList, FixedSizeList) and (FixedSizeList, LargeList)
• Float32 and Int64 inner type coercion
• Multi-row query with NULL row propagation
• Empty arrays (returns 0)
• No-args error
• Return-type assertion (Float64)

Are there any user-facing changes?

New scalar function inner_product, documented in docs/source/user-guide/sql/scalar_functions.md.

[Jefffrey]

Should we add a dot_product alias?

[crm26]

Thanks @Jefffrey — added dot_product as an alias in ef9895005, with SLT coverage for both a constant-args and a multi-row-with-NULL case. Doc regen picked up the alias automatically (#### Aliases block under inner_product, plus a top-level ### dot_product Alias of stub).

[alamb]

I had a thought about adding new functions:

• Add vector distance, array math, and array aggregate functions #21536 (comment)

[Jefffrey]

I think once merge conflict is fixed we should be good to merge this

[alamb]

I merged up to resolve a conflict

[alamb]

Thanks @crm26 and @Jefffrey