preprod live-test fixes: 4 real bugs surfaced in real-koios + chain integration

discovered during preprod smoke 2026-05-04 — 7 txs submitted (3 sends,
2 mints, 1 cip68 nft mint, 1 burn). all confirmed on chain. unit-test
coverage missed these because hand-crafted koios fixtures didn't match
real-world response shapes.

bugs:

PREPROD-1 (HIGH) — KoiosUtxo::asset_list deserializer rejected `null`.
real /address_utxos returns asset_list:null for ada-only utxos (vs
/address_info which returns []). Vec<T> can't deserialize null, killing
the entire utxo response. Option<Vec<T>>.unwrap_or_default fixes it +
new regression test deserializes_utxo_with_null_asset_list locks it in.

PREPROD-2 (HIGH) — /address_utxos needs `_extended: true` to populate
asset_list. without it, koios returns asset_list:[] (or null) for
asset-bearing utxos, making the wallet think it has zero of its own
tokens. native-asset send fails with "insufficient asset". new
AddressesExtendedBody serializer; get_utxos sets _extended=true.

PREPROD-3 (MEDIUM) — wallet_mint_cip68_nft default lovelace was 1.5 ADA
but the babbage min-utxo formula for inline-datum-bearing outputs
clears ~1.79 ADA. chain rejected with BabbageOutputTooSmallUTxO.
bumped default_token_lovelace 1_500_000 → 2_500_000 (covers typical
cip-68 metadata; large metadata still requires caller override).

PREPROD-4 (LOW, audit-process) — submit_tx error path called
.error_for_status() which discards koios's response body. chain-rule
rejections came through as bare HTTP codes, no diagnostic. now we
capture status + body before checking; rejections include the actual
ledger error (e.g. BabbageOutputTooSmallUTxO with the offending coin
amounts) so future debugging is one-shot.

7 successful preprod txs:
- e3e52cf9 self-send 3 ADA
- 397fe6b7 self-send 5 ADA via cold-sign flow (build_unsigned →
  tx_summary → sign_partial → submit_signed_tx; predicted tx_hash
  matched submitted tx_hash, body invariant under signing confirmed)
- d23e4c60 mint 100 ALDABRA_TEST with CIP-25 metadata
- 25cc489c mint cip-68 nft pair (ref label 100 + user label 222)
- 2ce72b6f mint 50 more ALDABRA_TEST via unsigned-mint flow
- 19a909df native-asset send (25 ALDABRA_TEST + 5 ADA)
- f949d29c burn 10 ALDABRA_TEST (negative-quantity mint)

guards verified:
- max_send_lovelace cap rejects 200 ADA without force ✓
- mint with insufficient holdings rejected with clear error ✓
- mcp tool names with dots silently dropped by Claude Code validator
  (already fixed in previous commit by renaming to underscore-only)

94 unit tests pass.

crates/aldabra-mcp/src/tools.rs

@@ -298,7 +298,14 @@ pub struct Cip68NftArgs {
 }
 
 fn default_token_lovelace() -> u64 {
-    1_500_000
+    // 2.5 ADA — Babbage min-utxo for an inline-datum-bearing
+    // multi-asset output is ~1.79 ADA (depends on datum size).
+    // 1.5 was too low; 2.5 gives comfortable margin for typical
+    // CIP-68 metadata (~150 bytes). Larger metadata still requires
+    // the caller to override.
+    // Discovered preprod 2026-05-04 via
+    // BabbageOutputTooSmallUTxO chain rejection.
+    2_500_000
 }
 
 #[derive(Debug, Deserialize, schemars::JsonSchema)]