wallet: Controller::generate_blocks now retries on "recoverable mempool errors"

[ImplOfAnImpl]

Got this failure on CI - https://github.com/mintlayer/mintlayer-core/actions/runs/25721713754/job/75524264465:

2026-05-12T08:53:17.2986831Z thread 'produce_blocks::case_1' panicked at wallet/wallet-cli-lib/tests/basic.rs:87:5:
2026-05-12T08:53:17.2987256Z assertion `left == right` failed
2026-05-12T08:53:17.2987894Z   left: "Wallet controller error: Node call error: Response error: ErrorObject { code: ServerError(-32000), message: \"Recoverable mempool error\", data: None }"
2026-05-12T08:53:17.2988509Z  right: "Success"

In that commit, basic.rs:87 corresponds to assert_eq!(test.exec("node-generate-blocks 20"), "Success"); and "Recoverable mempool error" means that while blockprod was collecting txs from the mempool, mempool's current tip changed and the whole block production attempt was aborted.
To address the problem, Controller::generate_blocks now checks for the "recoverable mempool error" case and retries after a delay.

There was also an inconsistency in mempool's collect_txs, which would return Ok(None) on the tip mismatch in one case (which indicates a "recoverable" error), and BlockConstructionError::TipMoved in another case. I removed the TipMoved error and now Ok(None) is returned in both cases.

P.S. I had another attempts to fix the problem in #2056, where generate_blocks would wait for NewTip event from the mempool instead. Neither solution is particularly elegant, but I ended up liking this one more.