[Bug]: theme dev proxy 502 on POST /account/login (regression of #3963?)

Environment

Summary

POST /account/login submitted to the local theme dev server (127.0.0.1:9292) fails at the CLI proxy with a 502 Bad Gateway, before the request reaches Shopify. This blocks any local automated flow (Playwright, WebdriverIO, curl) that needs a real customer session — the login form itself is unusable.

Looks like a possible regression of the (closed) #3963 which stated "It should work now no matter the version".

Error output

```
Failed to proxy request to /account/login with status 502 (Bad Gateway).
URL: https://.myshopify.com/account/login?_fd=0&pb=0

TypeError: fetch failed
at Object.processResponse (node:internal/deps/undici/undici:12793:20)
at node:internal/deps/undici/undici:13181:23
at process.processTicksAndRejections (node:internal/process/task_queues:104:5)
at node:internal/deps/undici/undici:17409:7
at process.processTicksAndRejections (node:internal/process/task_queues:104:5)
at async Object.handler (file:///.../@shopify/cli/dist/chunk-Q7BOSHWX.js:7:6357)
at async Server. (file:///.../@shopify/cli/dist/chunk-Q7BOSHWX.js:7:9903)
```

Repro

1. `shopify theme dev --store --theme --store-password `
2. From any HTTP client: `POST http://127.0.0.1:9292/account/login\` with the standard storefront login form fields.
3. The CLI logs the 502 above; the client sees a 502 response.

What we've tried

• Both Playwright's `page.request.post` and real form submission via a browser navigation. Both fail identically — the failure is at the proxy, not the client.
• Downgrading to `@shopify/cli@3.84.1`: this specific 502 does not occur, but the CLI then crashes with the (also known-broken) "Theme ID mismatch" bug during theme sync, so login can't complete for other reasons.
• Bypass attempt: POST direct to the primary storefront domain and transplant the resulting `_shopify_essential` cookie onto `127.0.0.1`. Theme dev issues a new `_shopify_essential` on every proxied response and overwrites the injected value, so the logged-in session never reaches Shopify.

Impact

Any team running functional/end-to-end tests against `theme dev` that need a real customer session cannot do so today. The workaround suggested in similar older threads — pushing to a real dev theme and hitting the preview URL — is unworkable at CI scale for stores that hit the theme cap.

Ask

Whether the proxy's `fetch failed` path is a known/tracked regression, or if there's a supported configuration we're missing to keep POST → cross-domain-redirect flows working through 4.6.0.

Start by reproducing the POST /account/login flow through shopify theme dev at 127.0.0.1:9292 using the documented store, theme, and password setup. Trace the theme dev proxy path around the reported fetch failed response and compare it with CLI 3.84.1. Done means the POST and its cross-domain redirect complete through the proxy without a 502 and preserve the customer session.