feat(solana): sendSponsoredSignedTransaction — sponsor + co-sign + broadcast a caller-built tx

[psmiratisu]

Adds a public `sendSponsoredSignedTransaction(serializedTxBase64)` to `PrivySolanaProviderAdapter` — EVM-paymaster parity for a server-built transaction (the trading planner's `solana-tx` legs).

Why

The existing sponsored path (`sendInstructions` → `sendSponsoredTransaction`) gates on `isAcpAction` (ACP program ids) and rebuilds the tx from instructions, so it can't sponsor a swap/bridge tx the trading planner already built. This method takes the prebuilt tx (user as placeholder fee payer) and runs the same sponsorship plumbing: `alchemy_requestFeePayer` (fee-payer swap + sig + CPI-rent prefund) → Privy user co-sign → broadcast + confirm, wrapped in the existing `withFeePayerRetry` / `minContextSlot` lag hardening.

• Sponsorship-only: no self-pay fallback — missing proxy/auth or sponsor refusal throws; the caller re-plans instead of silently billing the user's SOL.
• The tx's blockhash is fixed by the caller: retries re-run sponsor→sign→broadcast on the same bytes within the blockhash window; a stale blockhash surfaces as a terminal error to rebuild.

Proven live (mainnet)

tx `38NSvXFJsWkcRSA7xXBwVmXEY13vKwSjbm8qFmg6hMmkRqF1aYRfV6m4GzN6XnDhssy7uLkBUE93jcJsujSrouUw` — 2 sigs, fee payer `DmxEUYwgTX2qYnsf29LXBBScxYkKp1yn1Doodqf95SP7` (Alchemy), user paid 0 SOL, driven through acp-cli `feat/20260717` + the prod ACP proxy (`/wallets/solana-rpc/501`), which forwards `alchemy_requestFeePayer` for non-ACP txs.

Consumers

• acp-cli `feat/20260717` (`aa49860`): routes `sponsoredSubmit`-flagged trade legs through this method.
• trading-agent `430e1bf`: flags the leg + records the returned on-chain signature.

Merge this first — both consumers depend on it.

🤖 Generated with Claude Code

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Note

Medium Risk
New public submission path for sponsored Solana trades touches fee-payer RPC, signing, and confirmation/retry semantics; behavior is constrained by sponsorship-only errors and explicit non-retry on expiry for fixed transactions.

Overview
Adds sendSponsoredSignedTransaction on PrivySolanaProviderAdapter so a caller-built serialized transaction (planner swap/bridge legs with the user as placeholder fee payer) can go through the same sponsorship pipeline as instruction-based sends: alchemy_requestFeePayer → Privy co-sign → broadcast/confirm inside withFeePayerRetry, with no self-pay fallback if sponsorship is unavailable.

Because retries rebroadcast the same bytes (fixed blockhash), confirmation uses a lastValidBlockHeight bound resolved once across attempts (optional from the builder, otherwise from the first getLatestBlockhash), and retryExpired: false so an expired blockhash fails fast for the caller to rebuild.

withFeePayerRetry gains FeePayerRetryOptions.retryExpired (default unchanged): when false, SolanaTransactionError with phase "expired" is no longer retried—needed for fixed-blockhash senders while the instruction path keeps rebuilding with fresh blockhashes.

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