fix(sandbox-proxy): cap upstream response body size on all proxied routes

[pedrofrxncx]

Source: a hardening follow-up to #5316/#5319, found while auditing the same file (apps/api/src/api/routes/sandbox-proxy.ts) for this tick's "sandbox-proxy route robustness" focus area.

Why: #5316 and #5319 capped every route that reads a client request body fully into memory via c.req.text() before forwarding it to the daemon (forwardedBodyLimit, 10MB). The response side of the exact same routes has the identical gap: proxyDaemon's resolveAndFetch reads the daemon's response via upstream.text() with no size ceiling, fetchDaemonJson (used by suggest-commit/judge-review) does the same, and the preview-fetch/preview-invoke routes read a customer's own preview-server response the same unbounded way. A runaway git diff, a chatty exec script writing to stdout, or a large preview page gets buffered entirely into the shared Studio API process's memory — the same DoS-by-memory-exhaustion class #5316/#5319 just closed, just on the other side of the same fetch.

Fix: added readBoundedText(response, maxBytes), which reads a Response body in chunks via its reader and throws UpstreamPayloadTooLargeError once the running total exceeds maxBytes, instead of buffering an unbounded amount. Wired it into all four remaining upstream.text() call sites in this file, reusing the existing FORWARDED_BODY_MAX_BYTES (10MB) constant so the response cap matches the already-established request cap. proxyDaemon's catch block now returns a clear 502 for the new error instead of folding it into the generic "Daemon unreachable" message.

Regression test: readBoundedText returns the full body under the cap and throws UpstreamPayloadTooLargeError (instead of buffering) once a body exceeds it — added to sandbox-proxy.test.ts.

How to verify: bun test apps/api/src/api/routes/sandbox-proxy.test.ts (10 pass, including the 2 new cases); grep -n 'upstream.text()' apps/api/src/api/routes/sandbox-proxy.ts now returns nothing — every prior unbounded read goes through readBoundedText.

Checks run locally: bun run fmt and cd apps/api && bunx tsc --noEmit (both clean), plus the targeted test above. Full CI validates the rest.

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Summary by cubic

Cap upstream response body size to 10MB across all sandbox-proxy routes to prevent memory exhaustion. Responses are now read in chunks, with clear 502 errors when the cap is exceeded.

• Bug Fixes
  • Added readBoundedText(response, maxBytes) that streams and caps response bodies, throwing UpstreamPayloadTooLargeError.
  • Replaced all upstream.text() calls in apps/api/src/api/routes/sandbox-proxy.ts (proxyDaemon, fetchDaemonJson, preview-fetch, preview-invoke) with readBoundedText.
  • Reused FORWARDED_BODY_MAX_BYTES (10MB) to match the existing request body cap.
  • On oversize upstream bodies, return 502 with a specific error instead of the generic "Daemon unreachable".
  • Added tests for capped reads in sandbox-proxy.test.ts.

^{Written for commit 1e942fe. Summary will update on new commits.}