Add first-class in-process WordPress REST request runtime primitive

[chubes4]

Problem

wp-gym still works around REST replay by writing a temporary PHP wrapper that constructs WP_REST_Request, calls rest_do_request(), and parses the JSON output. That is generic WordPress runtime behavior, not an eval-specific concern.

The current Codebox http-response observation is useful for preview HTTP requests, but it does not replace the in-process REST execution path needed for deterministic WordPress API scenarios where callers need the same semantics as rest_do_request().

Desired Codebox primitive

Add a generic Codebox runtime command or observation, for example:

• wordpress.rest-request, or
• ObservationSpec.type = "wordpress-rest-response"

The primitive should accept:

• HTTP method.
• REST path, with /wp-json normalization handled generically.
• Headers.
• Optional request body.
• Optional response body inclusion policy.

It should return structured JSON with:

• status.
• headers.
• normalized response data/body.
• error diagnostics when WordPress returns or throws an error.
• artifact refs when response bodies are too large to inline.

Acceptance criteria

• A caller can execute a REST route inside the same disposable WordPress runtime without writing PHP wrapper files.
• The command/observation is generic and does not mention wp-gym, rewards, graders, scenarios, task sets, or model evals.
• Runtime policy gates the primitive like other WordPress runtime commands.
• Smoke coverage proves a registered REST route can be called and returns structured output.
• wp-gym can remove its stepRestWithCodeboxPhp() fallback after consuming this primitive.

Downstream evidence

Current workaround lives in wp-gym/src/index.js in stepRestWithCodeboxPhp().