fix(sandbox): cap the Go daemon's fs route request bodies

[pedrofrxncx]

Source: a real resource-bound gap, in the same lane as #6162 (which capped the daemon's outbound MCP JSON-RPC response body) — this caps the daemon's inbound fs request body, which was still unbounded.

Why a maintainer wants this: decodeBody in packages/sandbox/daemon-go/internal/routes/fs.go backs every /fs/* route (read, write, edit, mkdir, unlink, mv, list, write_from_url, upload_to_url) and read the request body with a plain io.ReadAll(r.Body) — no size cap. A caller sending an oversized body could buffer it entirely into the daemon's memory and crash it. Per this repo's own docs, the daemon's health probe is unforgiving: Studio marks the sandbox dead on a single missed probe and tears the pod down mid-session, so an OOM here is a real availability hit, not just a slow request.

Fix: cap the body at maxTransferBytes (500MB) — the bound the daemon already uses for its other file-transfer path (write_from_url/upload_to_url) — via the same io.LimitReader + explicit length check pattern #6162 used for the JSON-RPC response cap.

Regression test: TestDecodeBodyRejectsOversizedRequest in fs_test.go drives decodeBody with an infiniteReader (never hits EOF, no upfront allocation) and asserts it returns a size-limit error instead of buffering forever.

To verify: cd packages/sandbox/daemon-go && go test ./internal/routes/... -run TestDecodeBodyRejectsOversizedRequest -v

Checks run locally: go build ./..., go vet ./internal/routes/..., gofmt -l (clean), and the targeted test above (passes, ~0.5s). Full CI covers the rest.

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Summary by cubic

Caps the Go daemon’s /fs/* request bodies at 500MB to prevent OOM and sandbox teardown. Previously decodeBody read an unbounded body with io.ReadAll; now it wraps the body with io.LimitReader(maxTransferBytes+1) and rejects payloads over maxTransferBytes.

• Applies to all /fs/* routes (read, write, edit, mkdir, unlink, mv, list, write_from_url, upload_to_url); returns a size-limit error. Valid payloads are unaffected.
• Matches the existing 500MB transfer bound and mirrors the limit + explicit length check pattern from fix(sandbox): cap the Go daemon's MCP JSON-RPC response body #6162.
• Adds TestDecodeBodyRejectsOversizedRequest using an infinite reader to verify rejection without buffering.

^{Written for commit 11683c7. Summary will update on new commits.}