Fix UUID generation to guarantee full buffer fill using rand.Read

[Copilot]

Problem

The UUID generation code in common/uuid.go was using rand.Reader.Read() to fill the UUID buffer with random bytes. However, io.Reader.Read() does not guarantee that the entire buffer will be filled - it can return a short read without an error, potentially leaving parts of the UUID buffer uninitialized.

Solution

This PR replaces rand.Reader.Read(uuid) with rand.Read(uuid) in the NewUUID() function.

According to the Go documentation, rand.Read():

• Calls io.ReadFull() on rand.Reader internally
• Guarantees to fill the entire buffer completely
• Never returns an error (it panics on failure instead)
• Is the recommended way to get cryptographically secure random bytes

This matches our existing error handling behavior and ensures the UUID buffer is always completely filled with random data.

Changes

File: common/uuid.go

• Line 29: Changed from _, err := rand.Reader.Read(uuid) to _, err := rand.Read(uuid)

Testing

• ✅ All existing UUID tests pass (TestGUIDGenerationAndParsing)
• ✅ Common package tests verified
• ✅ Manually verified UUID generation produces valid, properly formatted UUIDs
• ✅ No other code in the repository affected by this change

Fixes the issue raised in #[issue_number] regarding rand.Reader.Read vs rand.Read.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>rand.Reader.Read vs rand.Read</issue_title>
<issue_description>Looking at the code in uuid.go (

azure-storage-azcopy/common/uuid.go

Line 29 in ec6313b

_, err := rand.Reader.Read(uuid)

):

I think it would be better to use something like:

_, err := io.ReadFull(rand.Reader, uuid)

or

_, err := rand.Read(uuid)

To guarantee filling the buffer (or returns a non-nil error)</issue_description>

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

Fixes #3248

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