fix(s3/transfermanager): respect MultipartUploadThreshold in upload decision

[nghiack7]

Problem

Options.MultipartUploadThreshold is documented and defaults to 16 MB, but the upload decision logic in nextReader() never reads it. The threshold used is PartSizeBytes (default 8 MB), so:

• Files between 8 MB and 16 MB are uploaded via multipart even though the caller expects a single PutObject for files below their configured threshold.
• Setting a custom MultipartUploadThreshold has no effect.

Reported in #3333.

Root Cause

// Before: threshold is PartSizeBytes, MultipartUploadThreshold ignored
r := io.LimitReader(u.in.Body, u.options.PartSizeBytes)
firstPart, _ := io.ReadAll(r)
if int64(len(firstPart)) < u.options.PartSizeBytes {
    return ..., io.EOF  // → PutObject
}
return ..., nil  // → multipart

Fix

Read up to max(MultipartUploadThreshold, PartSizeBytes) bytes. If all data fits within the threshold (EOF), use PutObject. Otherwise use multipart and re-inject any excess lookahead bytes back into the body via io.MultiReader so subsequent part reads remain at the normal PartSizeBytes granularity.

// After: use MultipartUploadThreshold as the decision boundary
readLimit := u.options.MultipartUploadThreshold
if u.options.PartSizeBytes > readLimit {
    readLimit = u.options.PartSizeBytes
}
lookahead, _ := io.ReadAll(io.LimitReader(u.in.Body, readLimit))
if int64(len(lookahead)) < u.options.MultipartUploadThreshold {
    return ..., io.EOF  // → PutObject
}
// re-inject overflow so part sizes stay at PartSizeBytes
if int64(len(lookahead)) > u.options.PartSizeBytes {
    overflow := lookahead[u.options.PartSizeBytes:]
    u.in.Body = io.MultiReader(bytes.NewReader(overflow), u.in.Body)
    lookahead = lookahead[:u.options.PartSizeBytes]
}
return ..., nil  // → multipart

Behaviour Changes

File size	Before	After
< PartSizeBytes (8 MB)	PutObject	PutObject
PartSizeBytes ≤ size < MultipartUploadThreshold (8–16 MB)	Multipart (bug)	PutObject (fixed)
≥ MultipartUploadThreshold (16 MB)	Multipart	Multipart
Custom MultipartUploadThreshold	Ignored	Respected

Part sizes for multipart uploads are unchanged (each part = PartSizeBytes).

Tests

Updated existing tests to reflect the corrected boundary. Added:

• TestUploadSingleBelowThreshold — 8 MB body uses PutObject with default 16 MB threshold
• TestUploadMultiAboveThreshold — 16 MB+1 body uses multipart with correct 8 MB+8 MB+1 parts
• TestUploadCustomThreshold — table-driven: below/at/above a 5 MB custom threshold

ok github.com/aws/aws-sdk-go-v2/feature/s3/transfermanager 7.2s