fix: deflake //rs/tests/ckbtc:ckbtc_minter_checker

[basvandijk]

//rs/tests/ckbtc:ckbtc_minter_checker has been slightly flaky in the last week:

$ bazel run //ci/githubstats:query -- top 1 flaky% --week --include //rs/tests/ckbtc:ckbtc_minter_checker
...
┍━━━━┯━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┯━━━━━━━━━┯━━━━━━━━━━━━━━━┯━━━━━━━━━┯━━━━━━━━━━━┯━━━━━━━━┯━━━━━━━━━━━━━━━━┯━━━━━━━━━━┯━━━━━━━━━━━━┯━━━━━━━━━┯━━━━━━━━━━┯━━━━━━━━━━━━━━━━━━┯━━━━━━━━━━━━━━━━┯━━━━━━━━━━┑
│    │ label                                 │   total │   non_success │   flaky │   timeout │   fail │   non_success% │   flaky% │   timeout% │   fail% │   impact │   total duration │   duration_p90 │ owners   │
┝━━━━┿━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┿━━━━━━━━━┿━━━━━━━━━━━━━━━┿━━━━━━━━━┿━━━━━━━━━━━┿━━━━━━━━┿━━━━━━━━━━━━━━━━┿━━━━━━━━━━┿━━━━━━━━━━━━┿━━━━━━━━━┿━━━━━━━━━━┿━━━━━━━━━━━━━━━━━━┿━━━━━━━━━━━━━━━━┿━━━━━━━━━━┥
│  0 │ //rs/tests/ckbtc:ckbtc_minter_checker │     123 │             3 │       3 │         0 │      0 │            2.4 │      2.4 │          0 │       0 │    25:03 │         17:07:03 │           8:21 │ defi     │
┕━━━━┷━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┷━━━━━━━━━┷━━━━━━━━━━━━━━━┷━━━━━━━━━┷━━━━━━━━━━━┷━━━━━━━━┷━━━━━━━━━━━━━━━━┷━━━━━━━━━━┷━━━━━━━━━━━━┷━━━━━━━━━┷━━━━━━━━━━┷━━━━━━━━━━━━━━━━━━┷━━━━━━━━━━━━━━━━┷━━━━━━━━━━┙

Failing due to, what on first sight seems to be, different reasons:

$ bazel run //ci/githubstats:query -- last --flaky --week  //rs/tests/ckbtc:ckbtc_minter_checker --columns=last_started_at,errors
...
╒════╤═════════════════════════╤══════════════════════════════════════════════════════════════════════════════════════════════════════════╕
│    │   last started at (UTC) │ errors per attempt                                                                                       │
╞════╪═════════════════════════╪══════════════════════════════════════════════════════════════════════════════════════════════════════════╡
│  0 │ Fri 2026-02-20 11:35:27 │ 1: -- Test timed out at 2026-02-20 11:35:26 UTC --                                                       │
├────┼─────────────────────────┼──────────────────────────────────────────────────────────────────────────────────────────────────────────┤
│  1 │ Wed 2026-02-18 17:27:59 │ 1: test_btc_checker: bug: could not send btc to btc client : JsonRpc(Rpc(RpcError { code: -26, messag... │
├────┼─────────────────────────┼──────────────────────────────────────────────────────────────────────────────────────────────────────────┤
│  2 │ Wed 2026-02-18 09:30:05 │ 1: test_btc_checker: called `Result::unwrap()` on an `Err` value: JsonRpc(Rpc(RpcError { code: -32603... │
╘════╧═════════════════════════╧══════════════════════════════════════════════════════════════════════════════════════════════════════════╛

Claude Opus 4.6 made the following root cause analysis and accompanying fix:

Root Cause

The RpcClient::send() method used ALL UTXOs from the sender address as transaction inputs, creating oversized transactions. After mining 101+ blocks to the wallet address, dozens of mature coinbase UTXOs were included, resulting in transactions whose minimum relay fee exceeded the hardcoded BITCOIN_NETWORK_TRANSFER_FEE of 1410 satoshis.

This caused three types of flaky failures:

1. min relay fee not met, 1410 < 5628 (RPC error -26): The fee was too low for the large transaction size.
2. ProcessNewBlock, block not accepted (RPC error -32603): Downstream effect of transaction pool issues.
3. Test timeout: Time wasted on failed operations and retries.

Fix

• Modified RpcClient::send() to select only the minimum UTXOs needed to cover amount + fee, keeping transactions small and fees adequate.
• Added retry logic (5 attempts with 2s delay) to send_to_btc_address() and generate_blocks() in test utils for transient RPC errors & made the functions idempotent.

See .claude/skills/fix-flaky-tests/SKILL.md.