ant-ffi external-signer: retry finalize after post-payment storage failure (needs ant-client#140)

[Nic-dorman]

Summary

Consumer-side follow-up to WithAutonomi/ant-client#140. Once ant-core makes a post-payment finalize failure recoverable, the ant-ffi external-signer surface must be updated to actually retain and retry the paid attempt instead of dropping it.

Background

The external-signer flow (PR #199) works like: prepare_* → external wallet pays payForQuotes → finalize_upload(upload_id, {quote_hash: tx_hash}). Today finalize_inner (ffi/rust/ant-ffi/src/client.rs) removes the PreparedUpload from the in-memory session map and passes it by value into ant-core's finalize_upload_with_progress, which consumes it. Tx-hash syntax is now validated before removal (bad input is retryable), but a genuine network/storage failure after payment still consumes the prepared state and strands the on-chain payment — see ant-client#140 for the root cause (quote hashes are timestamp+signature-derived, so re-prepare yields a non-matching tx map).

As of #199 this is documented as non-retryable after payment (the honest-contract stopgap). This issue tracks the real fix.

What needs doing here (after ant-client#140 lands)

1. Bump the ant-core rev pin in ffi/rust/ant-ffi/Cargo.toml (ant-core = { git = "...", rev = "..." }) to the commit carrying the retry-state API. Re-verify the ant-protocol = "2.2.2" pin still matches.
2. Retain the session on failure. In finalize_inner, stop discarding the upload when finalize fails post-payment. Keep the entry keyed by upload_id (or re-insert the returned retry-state) so a subsequent finalize_upload(upload_id, same_tx_map) retries storage of the unstored chunks against the same payment.
3. Update the docs/comments on finalize_upload / finalize_upload_with_progress and the sessions field to describe the new retryable contract (remove the "non-retryable after payment" language added in feat(ffi): re-add external-signer upload surface (wave-batch) #199).
4. Add a test exercising a simulated post-payment store failure → retry → success.

Links

• Root cause / core change: external-signer finalize: post-payment storage failure strands the payment (PreparedUpload consumed, not retryable) ant-client#140
• Surface added in: feat(ffi): re-add external-signer upload surface (wave-batch) #199