chore(rpc): adopt the prpc empty-body encoding for unit responses

[kvinwang]

Follow-up to #875, which dropped a transport-layer workaround that byte-matched null in
the Rocket responder. The problem is fixed properly upstream instead:
Phala-Network/prpc#1.

What changes

prpc-build 0.7.0 encodes a google.protobuf.Empty response as an empty JSON body
instead of the literal null. The decision is made in codegen from the response type, so
a method that legitimately returns a JSON null value is unaffected, and message fields of
type Empty still serialize as null inside the object — only the top-level body changes.
The protobuf codec is untouched; it has always encoded a unit as zero bytes.

Both RequestClient impls in the tree decode through prpc::codec::decode_json_from_slice,
which maps an empty body back to the unit type:

• ra-rpc::RaClient
• http-client::PrpcClient — missed in the first revision, caught in review. dstack-cli-core's
  stop_vm / remove_vm go through it and would have reported a failure for a request that
  actually succeeded. A compile check cannot catch this: serde_json::from_slice::<()>
  type-checks fine and only fails on the empty input at run time.

An empty body for any other message type stays an error.

Affected wire format

Every unit-returning JSON RPC now responds with an empty body rather than null:

• vmm: StartVm, StopVm, RemoveVm, ShutdownVm, ResizeVm, SvStop, SvRemove,
  PullRegistryImage, ...
• gateway: Exit, ReloadCert, and the admin RPCs
• kms: Onboard.Finish

Client audit:

Client	Status
ra-rpc::RaClient	fixed here
http-client::PrpcClient	fixed here
vmm web UI — vmmRpc (StartVm/StopVm/RemoveVm/ShutdownVm)	unaffected: application/octet-stream, protobuf path
vmm web UI — baseRpcCall (SvStop/SvRemove)	unaffected: never parses the response body
vmm-cli.py	unaffected: only parses a body that looks like JSON, otherwise returns it as a string
JS SDK send-rpc-request.ts	latent: bare JSON.parse(data) would throw, but guest-agent exposes no unit-returning method. Should be hardened before one is added

Verification

• cargo check / cargo clippy for kms, vmm, gateway, guest-agent, ra-rpc, http-client and
  dstack-cli-core: clean.
• cargo test -p ra-rpc: pass.
• Inspected the regenerated kms.rs / vmm.rs: dispatch_json_request emits
  Ok(Vec::new()) for unit methods, dispatch_request (protobuf) is byte-for-byte
  unchanged.

[Copilot]

Pull request overview

This PR updates dstack’s RPC stack to align JSON unit (google.protobuf.Empty / ()) responses with upstream prpc-build behavior (empty response body instead of literal null), and adjusts the ra-rpc client to decode that format correctly.

Changes:

• Bump prpc to 0.6.2 and prpc-build to 0.7.0 (workspace deps).
• Update ra-rpc’s RequestClient implementation to decode JSON responses via prpc::codec::decode_json_from_slice, which maps an empty body back to the unit type.
• Refresh Cargo.lock to reflect the dependency updates.

Reviewed changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated 1 comment.

File	Description
dstack/ra-rpc/src/client.rs	Switch JSON decoding to prpc::codec::decode_json_from_slice to support empty-body unit responses.
dstack/Cargo.toml	Bump workspace prpc / prpc-build versions to pick up the new unit-response encoding.
dstack/Cargo.lock	Lockfile update for the prpc / prpc-build version changes.

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[kvinwang]

Good catch, this was a real runtime break and my compile checks could not have caught it — serde_json::from_slice::<()> type-checks fine and only fails on the empty input at run time.

Fixed in 9887987: PrpcClient now decodes through prpc::codec::decode_json_from_slice as well. There are exactly two RequestClient impls in the tree (ra-rpc::RaClient and http-client::PrpcClient); both are updated now, and I left a comment at each call site so a future edit does not quietly revert to serde_json::from_slice.

[kvinwang]

Closing: the upstream change this depends on has been reverted.

prpc-build 0.7.0 and prpc 0.6.1 / 0.6.2 are yanked, and Phala-Network/prpc main is back at the pre-change commit. The JSON wire format for unit responses stays null.

Rationale: the empty-body encoding fixed nothing that was broken — the EOF bug in #875 was entirely fixed by the shutdown ordering, which is merged. What it bought was codec symmetry with protobuf; what it cost was a breaking wire change across 26 RPCs, a runtime break in PrpcClient that no compile check could catch, and lost boot-progress reporting for old CVM images on upgraded hosts. null is also valid JSON that every client can parse, which an empty body is not.