Code snippets: Support "Outputs:" comments in runnable snippets

[adamziel]

Core PR WordPress/wordpress-develop#13267 replaces a separate expected-output fence with a trailing output comment.

This PR adds support for the // Outputs: syntax to the runnable code snippets. That is in addition to the already supported expected-output syntax.

Output formats

Literal one-line output

echo esc_html( "<egg>" );
// Outputs: <egg>

Everything after // Outputs: is literal output. Quotes and other punctuation have no special meaning.

{
  "code": "echo esc_html( \"<egg>\" );",
  "expected_output": "<egg>"
}

Literal multiline output

print_r( array( "fruit" => "apple" ) );
// Outputs:
// Array
// (
//     [fruit] => apple
// )
//

An empty // Outputs: starts a multiline block. Each following // is one output line. One space after // is the comment delimiter; further indentation is output.

Empty final comments preserve final newlines. Quotes remain literal:

// Outputs:
// first
// "second "
//

This exports "first\n\"second \"\n".

JSON-encoded output

echo "done ";
// Outputs (JSON-encoded): "done "

// Outputs (JSON-encoded): requires one JSON string. Use it when otherwise invisible trailing whitespace, tabs, escaped quotes, or several encoded newlines must be explicit:

// Outputs (JSON-encoded): "first\nsecond \n\n"

The format is selected by the header, never by the output text. Unicode can remain literal in the DocBlock, including // Outputs (JSON-encoded): "✅ Complete ".

The expected-output fence remains supported for compatibility. It is still attached to the preceding interactive PHP fence and removed from rendered documentation. New examples should use one of the trailing comment forms above. The exported JSON shape stays the same, so consumers continue to receive expected_output as a string.

Testing

The expanded unit tests cover literal one-line and multiline output, literal quotes, indentation, trailing spaces, one and several final newlines, Unicode, valid JSON escapes, malformed JSON, non-string JSON values, non-final comments, strings, heredocs, block comments, same-line comments, text after a PHP closing tag, expected-output fence compatibility, output conflicts, and unattached metadata errors.

[github-actions]

Corpus diff

0 hunks. No behavior change over WordPress 7.1, parser at 0926d54 (base) vs 5c2a1f6 (PR head, merged into base).

[sirreal]

The old expected-output fence form is removed.

The two examples that shipped in WordPress 7.1 use this. I suspect it's OK to remove, but those examples will lose the expected output.

For 7.2, I'd expect to use whatever updated form is expected everywhere.