Try: Build WP once for PHPUnit test suite.

[peterwilsoncc]

Runs npm run build:dev in the newly created prepare-gutenberg reusable workflow for the PHPUnit tests.

This allows the step to be bypassed in each test, saving about 30 seconds of runtime.

As the local environment uses NPM for the docker pull and configuration, the npm ci step is still required. That can be a project for another time.

Trac ticket: https://core.trac.wordpress.org/ticket/65770

Use of AI Tools

AI assistance: Yes
Tool(s): GitHub Copilot
Used for: Initial drafting of this in another PR that also included some docker changes. The creation of this PR built upon portions of that but is much reduced in scope. AI was not used for the generation of this PR per se.

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[lancewillett]

This reusable workflow also serves test-coverage.yml, whose two coverage jobs do not pass wordpress-build-artifact

In this PR’s Code Coverage run, this step built WordPress in prepare-gutenberg, then both coverage jobs received an empty input and ran their fallback builds too. That adds a serial build and artifact upload without replacing either child build.

Could we pass wordpress-build from test-coverage.yml, or make this producer work opt-in?

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_{AI use: prepared with gpt-5.6-sol}