fix(deploy-key): require a key ID for delete

What does this MR do and why?

glab deploy-key delete treats its key ID as required everywhere except in argument validation:

Use:   "delete <key-id>",
Long:  `Pass the ID of the key to delete as an argument. ...
        This action is permanent and cannot be undone.`,
Example: `glab deploy-key delete 1234`,
Args:  cobra.MaximumNArgs(1),

MaximumNArgs(1) also accepts zero arguments, and complete only assigns keyID when exactly one is present:

if len(args) == 1 {
    ...
    o.keyID = int64(strInt)
}

So running the command with no argument leaves keyID at zero and sends DELETE /projects/:id/deploy_keys/0, then prints "Deploy key deleted." The command carries the Destructive MCP annotation, so it should not reach the API with an ID the user never supplied. There is no interactive fallback that would justify the optional argument.

cobra.ExactArgs(1) makes validation agree with the documented contract, and the new test case covers the no-argument path.

How to reproduce

glab deploy-key delete --repo group/project
# DELETE /api/v4/projects/group%2Fproject/deploy_keys/0
# Deploy key deleted.
# exit 0

Test coverage

Test_DeployKeyRemove gains a "Remove a deploy key without a key ID" case that asserts the argument error and registers no mock call, so a regression that reaches the API would fail the expectation.

Related issues

Closes #8410