fix(sandbox): cap the Go daemon's config-update body size

[pedrofrxncx]

Source: bug found while auditing the Go sandbox daemon (packages/sandbox/daemon-go/internal/routes) for reliability gaps — the same class already fixed for the file-transfer routes (fs.go's decodeBody, see #6215/#6039).

Payoff: ConfigUpdate (PUT /_sandbox/config) and OrgFsConfig both read the request body with a raw, unbounded io.ReadAll(r.Body). A misbehaving or malicious caller can stream an unbounded body into memory and crash the daemon — this is a single static binary per sandbox pod, and Studio tears the pod down on the next missed health probe, so an OOM here kills the whole session.

Failure scenario: POST a multi-GB body to /_sandbox/config or the org-fs config route → io.ReadAll buffers it all into memory with no limit → daemon OOMs/crashes → pod dies. fs.go already guards its routes against exactly this with a capped decodeBody; these two routes were missed.

Fix: added a shared readLimitedBody(r, maxBytes) helper in util.go (same pattern as fs.go's decodeBody, just returning raw bytes since these two callers each unmarshal differently) and wired both routes through it, capped at 1MB — config/org-fs payloads are always small hand-written JSON, never file transfers, so 1MB is generous headroom not a real ceiling.

Regression test: TestReadLimitedBodyRejectsOversizedRequest in util_test.go, mirroring the existing TestDecodeBodyRejectsOversizedRequest in fs_test.go — drives an infinite reader through the helper and asserts it's rejected instead of buffered.

To confirm: cd packages/sandbox/daemon-go && go build ./... && go vet ./... && go test ./internal/routes/...

Locally verified: go build, go vet, gofmt -l (clean), and the full internal/routes test package (including the existing config-redaction tests) — all green. CI validates the rest.

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Summary by cubic

Caps the request body for config endpoints at 1 MB to prevent daemon OOMs. Previously, ConfigUpdate and OrgFsConfig read unbounded bodies into memory; now oversized bodies are rejected with 400.

• Adds readLimitedBody in internal/routes/util.go and applies it to ConfigUpdate and OrgFsConfig.
• Sets maxConfigBytes to 1 MB; normal small JSON payloads are unaffected.
• Aligns config routes with the existing fs.go protection pattern.
• Adds TestReadLimitedBodyRejectsOversizedRequest to prevent regressions.

^{Written for commit de5fff6. Summary will update on new commits.}