wallet, rpc: accept an array of txids in removeprunedfunds #35660

Closes #29466.

removeprunedfunds only accepted a single txid, but the underlying CWallet::RemoveTxs already takes a vector and performs the removal in one atomic db transaction. This PR lets the RPC accept an array of txids so many transactions can be removed in a single wallet db transaction. A single txid string remains supported for backwards compatibility (via ParamFormat::JSON_OR_STRING in the bitcoin-cli conversion table, following the getblockstats hash_or_height precedent).

This adopts and reworks #29468 (by leo-aa88, credited as co-author) and addresses the feedback given there and in #32501:

• Backwards compatibility (raised in #29468): a plain string is still accepted, both positionally and as a named argument. The argument is renamed txid → txids; keeping a txid alias is not possible because rpc_help.py enforces that arguments named txid are plain strings API-wide.

• Performance justification (requested in #29468): benchmarked on regtest (macOS 26, arm64, SQLite wallet), removing imported pruned-funds transactions:

  | method | N=250 | N=1000 | |---|---|---| | individual RPC calls | 0.031s | 0.129s | | one JSON-RPC batch of individual calls | 0.011s | 0.048s | | one array call (this PR) | 0.001s | 0.005s |

  The JSON-RPC batch row addresses the "just use batch RPC calls" alternative raised in #29468: batching requests removes HTTP overhead, but the server still runs one wallet db txn per call — per the discussion in #29466, wallet db writes cannot be batched by batching RPC requests. The gains grow with wallet size and disk sync cost.

• Atomicity documented and tested: RemoveTxs runs within one db txn, so if any txid cannot be removed, none are. The functional test covers this (a failing array leaves the other transactions in place), plus empty-array and invalid-hex errors, and the legacy single-string form.

Testing done: wallet_importprunedfunds.py, wallet_resendwallettransactions.py, and rpc_help.py pass locally; all bitcoin-cli forms exercised manually (bare string, array of two, named txids=, empty array, invalid hex).

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35660.

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@furszy you offered in #29466 to review whenever a PR was open — tagging as requested.