fix(schedule): validate variables before creating a schedule

What does this MR do and why?

glab schedule create creates a real pipeline schedule before validating --variable values. An invalid value therefore makes the command return an error after the schedule already exists, leaving an orphan schedule without the requested variables.

The current code makes the side effect first:

schedule, _, err := client.PipelineSchedules.CreatePipelineSchedule(repo.FullName(), l)
// ...
for _, v := range variableList {
	split := strings.SplitN(v, ":", 2)
	if len(split) != 2 {
		return fmt.Errorf("invalid format for --variable: %s", v)
	}
}

This MR parses every variable into CreatePipelineScheduleVariableOptions before the first API call. If any value is invalid, no schedule or variable is created. Only after the whole list validates does it create the schedule and attach the parsed variables.

The touched client calls now also receive gitlab.WithContext(cmd.Context()), matching the repository's API client convention and allowing cancellation to propagate.

Test coverage

The existing invalid-variable case no longer expects CreatePipelineSchedule. A second case verifies that a valid first variable does not hide an invalid later value:

--variable foo:bar --variable invalid

On unmodified origin/main, both tests fail on the unexpected state-changing API call:

Unexpected call to CreatePipelineSchedule(...) because:
there are no expected calls of the method "CreatePipelineSchedule"

With the fix:

go test ./internal/commands/schedule/create -count=1
ok  gitlab.com/gitlab-org/cli/internal/commands/schedule/create

git diff --check also passes.

Related issues

Closes #8418 (closed)