[PHP] Use a contextual SAPI name

[adamziel]

Motivation for the change, related issues

Uses a relevant SAPI name for the current execution context:

• cli for CLI scripts
• playground for everything else

We've been struggling with the SAPI name for a while now. Some tools, like WP-CLI, expect the SAPI name to be cli – so we've hardcoded that. However, that broke other tools, such as the query-monitor plugin, that expect the SAPI name NOT to be cli.

Before this PR we could only set the SAPI name once – before the first PHP code execution. With this PR, we can set it at any point.

Implementation details

This PR ships a set_sapi_name( $new_name ) PHP function that updates the value returned by php_sapi_name() function and the PHP_SAPI constant.

Why a PHP runtime function? It's more convenient than controlling that from the TypeScript runtime via php.setSapiName(). We only need to maintain a single code path where we know the PHP runtime was already initialized. There's no need to check what if we're before the first code execution? Or what if we've trashed the runtime already? Or what do we do after the runtime hotswapping?

The .setSapiName() now sets the PLAYGROUND_SAPI_NAME constant, and the set_sapi_name() function is always called in the auto_prepend_file script with the current PLAYGROUND_SAPI_NAME constant value.

Testing Instructions (or ideally a Blueprint)

• The new CI unit tests should pass
• Confirm the query-monitor plugin continues to work: http://localhost:5400/website-server/?php=8.0&plugin=query-monitor
• Confirm the wp-cli step continues to work: http://localhost:5400/website-server/?php=8.0&blueprint-url=https://raw.githubusercontent.com/WordPress/blueprints/refs/heads/trunk/blueprints/posts-via-wp-cli/blueprint.json

[adamziel]

@johnbillion this PR will enable removing the recent query-monitor workaround