rpc: Align `getwalletinfo` response with the #55 decision (placeholder fields serialize as real zeros)

[pacu]

Current behavior: getwalletinfo responds with placeholder values: the handler logs TODO: Implement getwalletinfo and returns zero for balance, unconfirmed_balance, immature_balance, shielded_balance, shielded_unconfirmed_balance, txcount, keypoololdest, keypoolsize, walletversion: 0, and mnemonic_seedfp: "TODO". Only unlocked_until is computed (zallet-core/src/components/json_rpc/methods/get_wallet_info.rs).

#55 was closed (2026-07-13) with the decision that transparent balance data will not be added to getwalletinfo — users should use a dedicated balance retrieval method instead.

Problem: a caller today cannot distinguish "decided: use another method" from "balance is actually zero" — the placeholder fields serialize as real values. This is misleading for zcashd migrators whose monitoring reads getwalletinfo.balance.

Expected: align the response with the #55 decision:

1. Drop (or stop serializing) the balance fields that will never be populated, so callers fail loudly instead of reading zeros — or document their permanent absence.
2. Decide the fate of the remaining placeholder fields (walletversion, txcount, keypoololdest/keypoolsize — presumably 0 forever given no key pool, mnemonic_seedfp), and implement or remove them.
3. Record the resulting differences on the book's altered semantics page (the status matrix from book: Add a zcashd→Zallet RPC migration status matrix #602 already flags the method as partial).

Related: #55, #602, #600.

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Drafted with AI assistance (Claude); behavior verified against get_wallet_info.rs at a25cd85.

[linear]

COR-1582

[pacu]

Follow-up: traced the current handler (get_wallet_info.rs) field by field against what Zallet can actually source, so this is implementation-ready once the response shape is decided. Every field is a #55-style question of intent, not a missing query — grouping them:

A. Decided not to populate (per #55 — use dedicated balance methods):
balance, unconfirmed_balance, immature_balance, shielded_balance, shielded_unconfirmed_balance. These currently serialize as 0 / "0.00", which is the actual reported bug: a caller cannot distinguish "not provided" from "genuinely zero".

B. No Zallet analog — needs a keep/drop decision:

• walletversion — a Bitcoin-client integer version; currently 0. Zallet has no such concept.
• keypoololdest / keypoolsize — Zallet has no key pool (addresses are derived on demand, see the keypoolrefill note in the book). Arguably always-0 is correct here, but that should be a documented decision rather than a stub value.
• mnemonic_seedfp — the one clear bug regardless of shape: it returns the literal string "TODO". It is also semantically singular, but Zallet wallets hold multiple seeds (keystore.list_seed_fingerprints() returns a set). So this field cannot be faithfully represented as-is.

C. Correct today:
unlocked_until (computed from the keystore). txcount is sourceable (get_wallet_summary / a WalletRead count) but currently hardcoded to 0.

Recommended shape (for maintainer sign-off — this is the decision the method is blocked on):

1. Make the group-A balance fields Option and emit them as absent (skip_serializing_if) rather than 0, so the response is honest and forward-compatible; document the absence on the altered-semantics page. (Emitting null is the weaker alternative.)
2. Decide group B explicitly: drop walletversion/keypool* or document them as permanently 0; replace singular mnemonic_seedfp with either the wallet's set of fingerprints (new field, e.g. mnemonic_seedfps) or drop it in favour of z_listaccounts' seedfp.
3. Populate txcount from the wallet, and drop the warn!("TODO: Implement getwalletinfo").

Whichever shape is chosen is a serialization-critical change (per CONTRIBUTING) needing a CHANGELOG entry. Happy to implement once the group-A/B calls are made — I'd suggest (1) as the minimal, non-controversial first step since #55 already settled the balances direction.

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Investigated with AI assistance (Claude); field mapping verified against get_wallet_info.rs, keystore.rs, and database/connection.rs.