sdk/rs: retry JSON-RPC response errors that carry a transient status

[ayushsingh82]

Follows #4100 (which fixed this for Go) into the Rust SDK behind the doublezero CLI. #4098's core finding — Go and Rust disagreed on what's retryable — was only half resolved: the Rust predicate is_retryable_rpc_error (smartcontract/sdk/rs/src/client.rs) matched only ClientErrorKind::Io / Reqwest / Middleware and never looked at a decoded JSON-RPC error envelope.

So the two shapes the 2026-07-28 incident and malbeclabs/infra#2100 actually arrived in were still one-shot failures for every Rust CLI read:

• an HTTP status a provider load balancer put in the JSON-RPC code (RPCPool's 503 — the Go JSON-RPC retry classification is broken; Go and Rust disagree on what's retryable #4098 shape)
• transient wording with no machine-readable code (-32603 "Service unavailable, please try again later." from Helius — the infra#2100 shape)

Rust rode out 07-28 only because those particular 503s surfaced as Reqwest errors. Had they arrived as decoded envelopes, as they did for Go, every CLI invocation would have failed on the first attempt.

What changed

is_retryable_rpc_error now also classifies ClientErrorKind::RpcError, mirroring the Go classifier in tools/solana/pkg/jsonrpc:

• retry an envelope code that is really an HTTP status (429/500/502/503/504)
• retry the "busy / retry later" node codes -32005, -32004, -32429
• retry transient message wording (service unavailable, too many requests, bad gateway, gateway timeout, rate limited) when no recognizable code is set
• keep onchain / request-level rejections non-retryable: -32002 preflight, -32602, -32601, plus -32003 (signature-verification failure) and -32011 (no history) — deterministic, an identical retry gets the same answer

-32003 is listed as retryable in the issue text, but #4100 deliberately excludes it as deterministic; I followed #4100.

No send_transaction path in this SDK is wrapped with this predicate — every retry site is a read (get_account, get_program_accounts, get_signatures_for_address, …) — so an accepted-but-unacknowledged send is never resent. The classifier keeps that property and its doc comment states it.

Testing Verification

• cargo test -p doublezero_sdk — the new client_tests cases assert: an envelope 503/429 retries; -32005/-32004/-32429 retry; a -32603 whose message says "Service unavailable" retries while a -32603 "Internal error" does not; -32002/-32602/-32601/-32003/-32011 do not retry; transport (Io) still retries.

[ayushsingh82]

@ben-dz @nikw9944 — addresses #4101, the Rust-SDK parity half that #4098 flagged as still open. Classification here mirrors your merged Go fix in #4100 (tools/solana/pkg/jsonrpc) — tagging you two since you authored/reviewed that one and filed #4101.