fix(sandbox): cap the Go daemon's inline dispatch request body size

[pedrofrxncx]

Source: bounded resource fix (C3), same DoS-bound category as the already-merged #6215/#6039 fs caps and the open #6252 config-body cap — this is the /dispatch entry point, which none of those touch.

The gap: HandleDispatch in packages/sandbox/daemon-go/internal/dispatch/dispatch.go read the request body with a plain io.ReadAll(r.Body) — no size limit. Its sibling path, handleOffloadDispatch → FetchOffloadedMessages (offload.go), already caps external message fetches at maxOffloadBytes (32MB) with a Content-Length/read-limit check specifically because a large payload is expected to go through that offload path, not inline. The inline path had no equivalent bound, so one oversized POST to /dispatch could buffer arbitrarily large bytes into the pod's memory before validation even runs.

Fix: wrap the read in http.MaxBytesReader(w, r.Body, maxDispatchBodyBytes) (aliased to the existing maxOffloadBytes constant, so the two caps stay in sync) and answer 413 body_too_large via errors.As on *http.MaxBytesError instead of buffering past the cap.

Verification: TestDispatchRejectsOversizedBody sends a maxDispatchBodyBytes+1-byte body through HandleDispatch and asserts a 413. Ran go build ./..., go vet ./internal/dispatch/..., gofmt -l (clean), and go test ./internal/dispatch/... (full package, all green) locally — CI runs the rest.

A reviewer can confirm with: cd packages/sandbox/daemon-go && go test ./internal/dispatch/... -run TestDispatchRejectsOversizedBody -v.

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Summary by cubic

Caps the Go sandbox daemon’s inline /dispatch request body to prevent unbounded memory use. Previously io.ReadAll buffered the full body; now the read is limited to maxOffloadBytes (32MB) and oversized bodies return 413.

• Uses http.MaxBytesReader in HandleDispatch; on overflow returns 413 body_too_large before buffering. Other read/JSON errors still return 400 bad_json.
• Aligns the inline path with the offload path by aliasing the cap to maxOffloadBytes.
• Adds TestDispatchRejectsOversizedBody to assert the 413 response.

^{Written for commit 1572591. Summary will update on new commits.}