Harden iobuf implementation

[gmartin82]

Description

Harden _z_iosli_copy() implementation in the event of z_malloc() failing and make z_iosli_copy_bytes() copy data rather than append to match the name and current usage.

Why is this change needed?

To prevent unexpected behaviour when memory cannot be allocated and to make z_iosli_copy_bytes() behave as a copy as per its name.

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Labels: bug

🐛 Bug Fix Requirements

Since this PR is labeled as a bug fix, please ensure:

• Root cause documented - Explain what caused the bug in the PR description
• Reproduction test added - Test that fails on main branch without the fix
• Test passes with fix - The reproduction test passes with your changes
• Regression prevention - Test will catch if this bug reoccurs in the future
• Fix is minimal - Changes are focused only on fixing the bug
• Related bugs checked - Verified no similar bugs exist in related code

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