Fix theme dev failures with large storefront headers

[efegokdemir]

WHY are these changes introduced?

Fixes #8717

The theme dev proxy inherits Node and Undici header-size defaults. Cookie-heavy storefronts can exceed those defaults and return 502s.

WHAT is this pull request doing?

• Explicitly allow 128 KiB headers for storefront requests and the local theme dev server, including forwarded cookie headers.
• Preserve Shopify proxy environment settings with an Undici EnvHttpProxyAgent and show network error codes on the storefront error page.
• Add regression coverage for large upstream response headers, large local request headers, and error-code display, plus a theme patch changeset.

How to manually test your changes?

Run shopify theme dev for a storefront whose browser cookie jar produces more than 40 KiB of headers. Confirm storefront pages continue to render and localhost cookie forwarding does not return 502.