Fix manual_playable? to check for playable? jobs

What does this MR do and why?

Closes #29325

The play all button should only show when there are jobs within the stage that are playable. However, the button also appears when stages have only skipped jobs - in this case the the button does nothing and is useless and misleading.

This MR changes manually_playable? to check for playable? jobs within the stage so this useless button no longer shows.

Screenshots or screen recordings

Note the useless play all button in the deploy stage of the before screenshot. It is no longer there in the after screenshot.

Before	After

How to set up and validate locally

1. Be sure to set up runners for gdk https://gitlab.com/gitlab-org/gitlab-development-kit/blob/main/doc/howto/runner.md, this will enable you to run pipelines
2. Run gdk/gitpod server, login, navigate to Build > Pipeline Editor
3. Set up a pipeline (example ci config file below)
4. Trigger a pipeline by creating a MR
5. View a pipeline with skipped jobs in a stage - there should be no play all button. View a pipeline with manual jobs in a stage, there should still be a play all button.

Example ci config file with manual and skipped jobs:

workflow:
  rules:
    - if: $CI_PIPELINE_SOURCE == "merge_request_event"
    - if: $CI_COMMIT_BRANCH && $CI_OPEN_MERGE_REQUESTS
      when: never
    - when: always

default:
  image: busybox

stages:
  - stage1
  - stage2
  - stage3
  - stage4

job1:
  stage: stage1
  script:
    - export

manual-job1:
  stage: stage2
  script: echo
  when: manual

manual-job2:
  stage: stage2
  script: echo
  when: manual

job2:
  stage: stage3
  script:
    - exit 1

manual-job5:
  stage: stage4
  script: echo
  when: on_success

manual-job6:
  stage: stage4
  script: echo
  when: on_success

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.