Workflow fails to parse when 'star' and 'workflow_dispatch' triggers are combined: Unexpected value 'star'

[xbsheng]

Describe the bug

A workflow declaring both star and workflow_dispatch triggers fails to parse with:

failed to parse workflow: (Line: 3, Col: 3): Unexpected value 'star'

Each trigger alone parses fine. The combination fails regardless of order and regardless of whether types is specified.

Minimal repro

name: repro
on:
  star:
    types: [created]
  workflow_dispatch:
jobs:
  t:
    runs-on: ubuntu-latest
    steps:
      - run: echo hi

Attempting to dispatch it:

$ gh workflow run repro.yml
HTTP 422: Invalid Argument - failed to parse workflow: (Line: 3, Col: 3): Unexpected value 'star'

Test matrix (all verified against github.com via the dispatch API):

triggers	result
star: with types: [created], alone	parses
workflow_dispatch:, alone	parses
star: + fork:	parses
push: + workflow_dispatch:	parses
star: + workflow_dispatch:	fails
workflow_dispatch: then star: (reversed)	fails
star: (no types) + workflow_dispatch:	fails

Expected behavior: both triggers are valid events individually, so the combination should parse like any other event combination.

Workaround: use watch: types: [started] instead of star (starring a repository fires the watch event with action: started). This parses and works correctly.

Environment: github.com; workflow first added to the default branch also shows a failed run (0s, no jobs, listed by file path) for the push that added it.