Fix preview proxy REST origin rewriting

[chubes4]

Summary

• rewrite the exact upstream Playground origin to the local preview origin in textual proxy responses and response headers
• stream rewrites with bounded memory while preserving serialized FIFO upstream requests and abort behavior
• cover Gutenberg-style REST preflight and mutation forwarding through the corrected same-origin proxy contract

Fixes #2095

Evidence

• Source relationship: fixes Preview proxy blocks Gutenberg editor saves on CORS preflight #2095 at the local preview proxy, where WordPress-generated absolute upstream URLs escaped the browser-visible proxy origin.
• Change kind: bounded runtime proxy correction plus deterministic proxy/editor contract coverage.
• Verification capability: the proxy test serves a chunk-split, JSON-escaped upstream REST URL, proves it becomes proxy-origin, forwards an OPTIONS request carrying x-http-method-override, and then completes the proxied REST mutation with its body and override header intact. Existing concurrency and aborted-request assertions remain in the same passing contract.
• Scope: rewriting is limited to the proxy's exact configured upstream origin in identity-encoded textual bodies and response header values. It does not special-case Gutenberg, REST route names, or plugins, and it does not allow additional origins or headers. Encoded/binary responses and request forwarding remain unchanged.

Verification

• TMPDIR=/tmp npm run test:browser-callback-materialization-contracts
• TMPDIR=/tmp npm run test:editor-actions
• npm run build
• git diff --check

The editor test fixture was updated to include the runtime environment now required by the shared PHP bootstrap; no editor runtime behavior changed.

AI Assistance Disclosure

• AI assistance: Yes
• Tool(s): OpenCode and Homeboy
• Model: openai/gpt-5.6-sol
• Used for: Root-cause tracing, bounded proxy implementation, deterministic contract coverage, verification, and PR preparation. Chris Huber remains responsible for the submitted change.