Improve socket error detection in unicast transport read

[sashacmc]

Description

What does this PR do?

Improves error detection and handling in the unicast transport read path:

• Enhanced socket error detection to distinguish between pending data, socket closure, and critical errors
• Proper handling of socket closure events to allow reconnection
• Changed _z_unicast_client_read return type from bool to int with distinct status codes
• Exit read task when connection errors are detected instead of continuing to loop

Why is this change needed?

Previously, the unicast read function returned a simple boolean, which didn't distinguish between "no data available" and "socket closed/error". This could lead to:

• Read task continuing to loop indefinitely on closed sockets
• Inability to properly trigger reconnection logic
• Unclear error states that made debugging difficult

This change enables proper reconnection handling by detecting when the socket is actually closed or experiencing critical errors.

Related Issues

Fixes #1010

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🏷️ Label-Based Checklist

Based on the labels applied to this PR, please complete these additional requirements:

Labels: enhancement

✨ Enhancement Requirements

Since this PR enhances existing functionality:

• Enhancement scope documented - Clear description of what is being improved
• Minimum necessary code - Implementation is as simple as possible, doesn't overcomplicate the system
• Backwards compatible - Existing code/APIs still work unchanged
• No new APIs added - Only improving existing functionality
• Tests updated - Existing tests pass, new test cases added if needed
• Performance improvement measured - If applicable, before/after metrics provided
• Documentation updated - Existing docs updated to reflect improvements
• User impact documented - How users benefit from this enhancement

Remember: Enhancements should not introduce new APIs or breaking changes.

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