[Repo Assist] Add AsyncSeq.reduce and AsyncSeq.reduceAsync

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Summary

Adds two new aggregation combinators that mirror Seq.reduce:

AsyncSeq.reduce    : reduction:('T -> 'T -> 'T)          -> source:AsyncSeq<'T> -> Async<'T>
AsyncSeq.reduceAsync : reduction:('T -> 'T -> Async<'T>) -> source:AsyncSeq<'T> -> Async<'T>

Usage:

// Sum an async sequence without an initial state
let! total = asyncSeq { yield 1; yield 2; yield 3 } |> AsyncSeq.reduce (+)
// total = 6

// Custom async reduction
let! result =
    asyncSeq { yield "hello"; yield " "; yield "world" }
    |> AsyncSeq.reduceAsync (fun a b -> async { return a + b })
// result = "hello world"

Motivation

Seq.reduce is a standard F# function that is commonly used when no natural "zero" element exists (e.g. max, string concatenation, non-commutative operations). Its absence from AsyncSeq is a minor but consistent API gap. fold/foldAsync can work around it by passing the first element as the state, but that's awkward and easy to get wrong.

Implementation

• reduceAsync consumes the internal pull-based enumerator directly, yielding the first element as the initial accumulator.
• reduce delegates to reduceAsync (same pattern as fold/foldAsync).
• Raises InvalidOperationException("The input sequence was empty.") when the source is empty, matching Seq.reduce semantics.

Test Status

✅ All 199 tests pass (dotnet test). 5 new tests cover:

1. Non-empty sequence sums correctly
2. Single-element sequence returns that element unchanged
3. Empty sequence raises InvalidOperationException
4. Async reduction function works correctly
5. Results match Seq.reduce across multiple input sizes

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[github-actions]

Pull request created: #242