Fix wp-env deprecation warning: separate dev/test configs and clean up formatting

[Copilot]

@wordpress/env v11+ deprecates testsEnvironment, causing warnings. This PR introduces a dedicated .wp-env.tests.json for the test stack and removes the deprecated option from both configs.

Changes

• .wp-env.json — 2-space indentation (per .editorconfig); removed deprecated testsEnvironment: false
• .wp-env.tests.json — 2-space indentation; removed deprecated testsEnvironment: false
• package.json — Added wp-env:start:tests / wp-env:stop:tests scripts and updated PHP tooling scripts to use --config=.wp-env.tests.json
• .github/workflows/php-test.yml — Uses dedicated wp-env:start:tests npm script (with -- --xdebug=coverage for coverage runs) instead of inline --config flag, keeping CI/local behavior consistent

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Co-authored-by: ernilambar <nilambar@git.wordpress.org>

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