Fix chunk index mismatch between getSliceChunkFile and checkChunkSize

[Copilot]

checkChunkSize was called with idx (reqChunks[from]) but validated against a file opened for availableChunks[index]. These differ when the anchor chunk found via binary search isn't the originally requested chunk — causing false size-mismatch errors, particularly for the last chunk which has a smaller expected size.

// Before: idx is the requested chunk, but chunkFile was opened for availableChunks[index]
chunkFile, _ := getSliceChunkFile(c, availableChunks[index])
if err := checkChunkSize(c, chunkFile, idx); err != nil {

// After: pass the actual chunk index matching the opened file
if err := checkChunkSize(c, chunkFile, availableChunks[index]); err != nil {

• Fix: Pass availableChunks[index] to checkChunkSize to match the chunk file that was actually opened
• Test: Added Test_getContents_lastChunkSizeMismatch — reproduces the bug using a partial last chunk (100KB vs 512KB block size) where the requested and anchor chunk indices diverge

Original prompt

---

This section details on the original issue you should resolve

<issue_title>getSliceChunkFile and checkChunkSize may be not match</issue_title>
<issue_description>```
code in getContents like:
fromByte := uint64(reqChunks[from] * uint32(c.md.BlockSize))
if index < len(availableChunks) {
chunkFile, _ := getSliceChunkFile(c, availableChunks[index])
if chunkFile != nil {
if err := checkChunkSize(c, chunkFile, idx); err != nil {
_ = chunkFile.Close()
_ = c.bucket.Discard(context.Background(), c.id)
return nil, 0, err
}

chunkFile, _ := getSliceChunkFile(c, availableChunks[index]
checkChunkSize(c, chunkFile, idx)

the question is here: 
`		chunkFile, _ := getSliceChunkFile(c, availableChunks[index])
		if chunkFile != nil {
			if err := checkChunkSize(c, chunkFile, idx`
idx != availableChunks[index]</issue_description>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes getSliceChunkFile and checkChunkSize may be not match #48

---

💬 Send tasks to Copilot coding agent from Slack and Teams to turn conversations into code. Copilot posts an update in your thread when it's finished.

[Copilot]

Pull request overview

Fixes an incorrect chunk-index being passed to checkChunkSize in the caching middleware, which could trigger false “size mismatch” errors when the anchor chunk found via binary search differs from the originally requested chunk (notably for a partial last chunk).

Changes:

• Pass availableChunks[index] (the chunk actually opened) into checkChunkSize in getContents.
• Add a regression test covering the partial last-chunk mismatch scenario.
• Minor related refactors/adjustments (iobuf helper/parameter rename) and module dependency bumps.

Reviewed changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
server/middleware/caching/internal.go	Fixes the checkChunkSize index mismatch when using an anchor chunk; adjusts how the combined reader is constructed.
server/middleware/caching/caching_chunkpart_test.go	Adds a test reproducing the last-chunk partial size mismatch scenario.
pkg/iobuf/part_reader.go	Renames the PartsReadCloser parameter to clarify “optional closer” intent (no functional change).
pkg/iobuf/closer_reader.go	Adds NopCloser() helper used by the caching code path.
go.mod	Bumps multiple dependency versions.
go.sum	Updates checksums corresponding to the dependency bumps.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

You can also share your feedback on Copilot code review. Take the survey.