fix(objectstore): guard credential secret against unsafe args[0] indexing and reject extra args

[karthik120710]

Description

close #601

objectStoreCredentialSecretCmd resolved its access key with:

var key string
if accessKey != "" {
    key = accessKey
} else if args[0] != "" {
    key = args[0]
}

args[0] was read without first checking len(args), and the command had no Args: validator. In the current code this is not reachable at runtime — --access-key is already registered as a required flag (MarkFlagRequired), so cobra's flag validation rejects a missing key before Run ever executes — but the code was still fragile: it panics with an index-out-of-range error the moment that required-flag safeguard is removed or the command is reused elsewhere without it, and there was no test coverage protecting the key-resolution logic itself.

Changes

- Extracted the key-resolution logic into a small pure function, resolveAccessKey(accessKey string, args []string) string, which returns the access key: the --access-key flag takes precedence, otherwise the first positional argument is used if present, otherwise "".
- Replaced the unsafe args[0] != "" check with len(args) > 0 before indexing.
- Added Args: cobra.MaximumNArgs(1) to the command so a stray extra positional argument (e.g. civo objectstore credential secret a b) is now rejected with a clear cobra usage error instead of being silently ignored.
- Added objectstore_credential_secret_test.go with table-driven tests covering: no flag/no args, positional-arg-only, flag-only, and flag-takes-precedence-over-positional-arg.

[karthik120710]

hi @giornetta @hlts2 i have fix the potential bug and added test case to verify my changes can u review my pr.

[giornetta]

hi @giornetta @hlts2 i have fix the potential bug and added test case to verify my changes can u review my pr.

Thanks for the PR! I'll review it as soon as possible 🙌