config: reject names containing slashes

[ousamabenyounes]

What

Reject configuration names containing / when adding a store, source, or destination configuration.

This prevents commands like plakar destination add s3://xxxx access_key=yyy from silently creating a destination named s3://xxxx instead of reporting the missing name.

Closes #2102

Test verification (RED -> GREEN)

RED on origin/main plus the new test, in Docker (golang:1.25):

--- FAIL: TestDispatchAddRejectsNameWithSlash (0.00s)
    config_extra_test.go:151:
        Error:       An error is expected but got nil.
FAIL
FAIL github.com/PlakarKorp/plakar/subcommands/config 0.006s

GREEN after the fix, same targeted Docker test:

ok github.com/PlakarKorp/plakar/subcommands/config 0.008s

Package validation:

ok github.com/PlakarKorp/plakar/subcommands/config 0.069s

Full validation note

Docker go build -v ./... completed successfully. Docker go test ./... then failed in subcommands/pkg with a Pebble closed-cache panic in TestPkgCreateExecuteFailsOnBadConnector:

panic: pebble: closed
FAIL github.com/PlakarKorp/plakar/subcommands/pkg 0.263s
FAIL

I checked that failure against clean origin/main; the same test also fails there in the same Docker non-root environment:

=== RUN   TestPkgCreateExecuteFailsOnBadConnector
panic: pebble/record: closed LogWriter
FAIL github.com/PlakarKorp/plakar/subcommands/pkg 0.141s
FAIL

[omar-polo]

Hello,

thanks for the fix! to be 100% clear, I think the PR is slightly more complex than what is needed, but it definitely fixes the issue and adds a test even, which is very appreciated! I'll merged it as-is and fixup a style nit as a follow-up.