feat(server): Implement and document security group attach/detach commands

[s-inter]

Description

This PR implements the stackit server security-group attach and detach commands.

Changes

• Added stackit server security-group attach to associate a security group with a server.
• Added stackit server security-group detach to remove a security group from a server.
• Added unit tests for input parsing and request building.
• Generated updated command documentation.

Testing Instructions

1. Preconditions: have an existing server and a security group in the same project/region.
2. Build:

make build

3. Attach:

./bin/stackit server security-group attach --server-id <SERVER_ID> --security-group-id <SG_ID>

3. Verify attach via server describe:

./bin/stackit server describe <SERVER_ID> --output-format json | jq '[.nics[].securityGroups[]]'

Confirm the security group appears in the server details.

4. Detach:

./bin/stackit server security-group detach --server-id <SERVER_ID> --security-group-id <SG_ID>

5. Verify detach via server describe and confirm the security group is no longer associated.

Related Issues

Resolves #1216

relates to STACKITCLI-308

Checklist

• Issue was linked above
• Code format was applied: make fmt
• Examples were added / adjusted (see e.g. here)
• Docs are up-to-date: make generate-docs (will be checked by CI)
• Unit tests got implemented or updated
• Unit tests are passing: make test (will be checked by CI)
• No linter issues: make lint (will be checked by CI)

[github-actions]

Merging this branch will increase overall coverage

Impacted Packages	Coverage Δ	🤖
github.com/stackitcloud/stackit-cli/internal/cmd/server	0.00% (ø)
github.com/stackitcloud/stackit-cli/internal/cmd/server/security-group	0.00% (ø)
github.com/stackitcloud/stackit-cli/internal/cmd/server/security-group/attach	36.59% (+36.59%)	🌟
github.com/stackitcloud/stackit-cli/internal/cmd/server/security-group/detach	36.59% (+36.59%)	🌟
github.com/stackitcloud/stackit-cli/internal/cmd/server/service-account/detach	45.00% (ø)

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Coverage by file

Changed files (no unit tests)

Changed File	Coverage Δ	Total	Covered	Missed	🤖
github.com/stackitcloud/stackit-cli/internal/cmd/server/security-group/attach/attach.go	36.59% (+36.59%)	41 (+41)	15 (+15)	26 (+26)	🌟
github.com/stackitcloud/stackit-cli/internal/cmd/server/security-group/detach/detach.go	36.59% (+36.59%)	41 (+41)	15 (+15)	26 (+26)	🌟
github.com/stackitcloud/stackit-cli/internal/cmd/server/security-group/security-group.go	0.00% (ø)	5 (+5)	0	5 (+5)
github.com/stackitcloud/stackit-cli/internal/cmd/server/server.go	0.00% (ø)	26 (+1)	0	26 (+1)
github.com/stackitcloud/stackit-cli/internal/cmd/server/service-account/detach/detach.go	45.00% (ø)	40	18	22

Please note that the "Total", "Covered", and "Missed" counts above refer to code statements instead of lines of code. The value in brackets refers to the test coverage of that file in the old version of the code.

Changed unit test files

• github.com/stackitcloud/stackit-cli/internal/cmd/server/security-group/attach/attach_test.go
• github.com/stackitcloud/stackit-cli/internal/cmd/server/security-group/detach/detach_test.go

[s-inter]

Typo fix (boyscout) - okay to keep in here?