fix(s3/transfermanager): use MultipartUploadThreshold for upload decision

[abhu85]

Summary

Fixes #3333 - MultipartUploadThreshold was defined but never used in upload decision logic.

Problem

The MultipartUploadThreshold field in transfermanager.Options was defined and resolved with a default value (16 MiB), but it was never actually referenced in the upload decision logic. The single-upload vs multipart-upload decision was solely determined by PartSizeBytes, making MultipartUploadThreshold dead code with no effect on behavior.

Solution

Use min(MultipartUploadThreshold, PartSizeBytes) as the cutoff for single vs multipart upload decision in nextReader(). This ensures:

1. MultipartUploadThreshold now works: Setting a lower threshold (e.g., 1KB) will trigger multipart uploads for smaller files
2. Backward compatibility: With default values (PartSizeBytes=8MB, MultipartUploadThreshold=16MB), the effective threshold becomes 8MB - identical to current behavior since the threshold was previously dead code
3. No data truncation: The threshold is capped to PartSizeBytes since we can only observe that much data in the first read

Changes

• api_op_UploadObject.go: Modified nextReader() to use threshold-based decision
• api_op_UploadObject_test.go: Added 3 new tests for threshold behavior

Test Plan

• TestUploadMultipartTriggeredByThreshold - verifies low threshold triggers multipart
• TestUploadSingleWhenBelowThreshold - verifies files below threshold use single upload
• TestUploadThresholdCappedByPartSize - verifies backward compatibility when threshold > partSize
• All 33 existing upload tests pass
• Full transfermanager test suite passes

Reproduction

Before fix:

// Set threshold to 1KB — expect multipart upload for anything > 1KB
tm := transfermanager.New(s3Client, func(o *transfermanager.Options) {
    o.MultipartUploadThreshold = 1024 // 1 KB
    o.PartSizeBytes = 5 * 1024 * 1024 // 5 MB
})
// Upload 100KB — SHOULD use multipart (> 1KB threshold)
// but ACTUALLY uses single PutObject (< 5MB PartSizeBytes)

After fix:

// Same config — now works as expected
// 100KB upload uses multipart because 100KB > 1KB threshold

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