app: Update socket AT commands to use handle-based API

[juhaylinen]

All socket operations now require a handle obtained from #XSOCKET or #XSSOCKET command. This allows more flexible way to manage sockets.

Remove AT command #XSOCKETSELECT.
Add AT command #XCLOSE to close individual socket or all sockets at once.

Update documentation.

SLM-50

[Copilot]

Pull Request Overview

This PR updates socket AT commands to use a handle-based API instead of socket selection, providing more flexible socket management by directly referencing socket handles in all operations.

• Removes AT#XSOCKETSELECT command and socket ranking system
• Adds AT#XCLOSE command for individual or bulk socket closure
• Updates all socket operations to require handle parameters

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
doc/app/sm_migration.rst	Adds migration guide documenting the transition from socket selection to handle-based API
doc/app/SOCKET_AT_commands.rst	Updates command documentation to reflect handle-based parameters and new #XCLOSE command
app/src/sm_at_socket.c	Implements handle-based socket operations, removes socket selection logic, and adds close command handler

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[MarkusLassila]

Moving into good direction!

I'll review the documentation after the sm_at_socket.c is in order.

[juhaylinen]

@MarkusLassila @trantanen I addressed the findings. Please re-review.

[MarkusLassila]

Some nits, but LGTM!

[MarkusLassila]

@juhaylinen: Handle these github actions complaints.

[trantanen]

Indeed, compliance has several warnings about long lines but the check is showing green in github. Compliance log has these long lines as warnings, not errors so it doesn't fail. I will need to look at this but meanwhile, please fix the failures.

[MarkusLassila]

Merge away.

@moksanen && @jvaliharju, prepare for breaking socket tests.