Merge bitcoin/bitcoin#29735: AcceptMultipleTransactions: Fix workspace not being set as client_maxfeerate failure

4ba1d0b55339c3ea90e2bcd64662a06f0f90dd46 fuzz: Add coverage for client_maxfeerate (Greg Sanders)
91d7d8f22a1c528db14fa743c66cd861ea00e84b AcceptMultipleTransactions: Fix workspace client_maxfeerate (Greg Sanders)
f3aa5bd5eb6d1088f98a4dc7daaab0e17a7d5529 fill_mempool: assertions and docsctring update (Greg Sanders)
a3da63e8febe475f2250f6432bca237d31fa9107 Move fill_mempool to util function (Greg Sanders)
73b68bd8b4f9447e30091c7f8c3dc91a086bd93b fill_mempool: remove subtest-specific comment (Greg Sanders)

Pull request description:

  Bug causes an `Assume()` failure due to the expectation that the individual result should be invalid when done over `submitpackage` via rpc.

  Bug introduced by https://github.com/bitcoin/bitcoin/pull/28950 , and I discovered it rebasing https://github.com/bitcoin/bitcoin/pull/28984 since it's easier to hit in that test scenario.

  Tests in place were only checking `AcceptSingleTransaction`-level checks due to package evaluation only triggering when minfee is too high for the parent transaction.

  Added test along with fix, moving the fill_mempool utility into a common area for re-use.

ACKs for top commit:
  glozow:
    reACK 4ba1d0b55339c3ea90e2bcd64662a06f0f90dd46
  theStack:
    ACK 4ba1d0b55339c3ea90e2bcd64662a06f0f90dd46
  ismaelsadeeq:
    re-ACK https://github.com/bitcoin/bitcoin/commit/4ba1d0b55339c3ea90e2bcd64662a06f0f90dd46  via [diff](https://github.com/bitcoin/bitcoin/compare/4fe7d150eb3c85a6597d8fc910fe1490358197ad..4ba1d0b55339c3ea90e2bcd64662a06f0f90dd46)

Tree-SHA512: 3729bdf7f25d04e232f173ccee04ddbb2afdaafa3d04292a01cecf58fb11b3b2bc133e8490277f1a67622b62d17929c242dc980f9bb647896beea4332ee35306

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