Merge bitcoin/bitcoin#26289: Use util::Result in for calculating mempool ancestors

47c4b1f52ab8d95d7deef83050bad49d1e3e5990 mempool: log/halt when CalculateMemPoolAncestors fails unexpectedly (stickies-v)
5481f65849313ff947f38433b1ac28285a7f7694 mempool: add AssumeCalculateMemPoolAncestors helper function (stickies-v)
f911bdfff95eba3793fffaf71a31cc8bfc6f80c9 mempool: use util::Result for CalculateMemPoolAncestors (stickies-v)
66e028f7399b6511f9b73b1cef54b6a6ac38a024 mempool: use util::Result for CalculateAncestorsAndCheckLimits (stickies-v)

Pull request description:

  Upon reviewing the documentation for `CTxMemPool::CalculateMemPoolAncestors`, I noticed `setAncestors` was meant to be an `out` parameter but actually is an `in,out` parameter, as can be observed by adding `assert(setAncestors.empty());` as the first line in the function and running `make check`. This PR fixes this unexpected behaviour and introduces refactoring improvements to make intents and effects of the code more clear.

  ## Unexpected behaviour
  This behaviour occurs only in the package acceptance path, currently only triggered by `testmempoolaccept` and `submitpackage` RPCs.

  In `MemPoolAccept::AcceptMultipleTransactions()`, we first call `PreChecks()` and then `SubmitPackage()` with the same `Workspace ws` reference. `PreChecks` leaves `ws.m_ancestors` in a potentially non-empty state, before it is passed on to `MemPoolAccept::SubmitPackage`. `SubmitPackage` is the only place where `setAncestors` isn't guaranteed to be empty before calling `CalculateMemPoolAncestors`. The most straightforward fix is to just forcefully clear `setAncestors` at the beginning of CalculateMemPoolAncestors, which is done in the first bugfix commit.

  ## Improvements
  ### Return value instead of out-parameters
  This PR updates the function signatures for `CTxMemPool::CalculateMemPoolAncestors` and `CTxMemPool::CalculateAncestorsAndCheckLimits` to use a `util::Result` return type and eliminate both the `setAncestors` `in,out`-parameter as well as the error string. It simplifies the code and makes the intent and effects more explicit.

  ### Observability
  There are 7 instances where we currently call `CalculateMemPoolAncestors` without actually checking if the function succeeded because we assume that it can't fail, such as in [miner.cpp](https://github.com/bitcoin/bitcoin/blob/69b10212ea5370606c7a5aa500a70c36b4cbb58f/src/node/miner.cpp#L399). This PR adds a new wrapper `AssumeCalculateMemPoolAncestors` function that logs such unexpected failures, or in case of debug builds even halts the program. It's not crucial to the objective, more of an observability improvement that seems sensible to add on here.

ACKs for top commit:
  achow101:
    ACK 47c4b1f52ab8d95d7deef83050bad49d1e3e5990
  w0xlt:
    ACK https://github.com/bitcoin/bitcoin/pull/26289/commits/47c4b1f52ab8d95d7deef83050bad49d1e3e5990
  glozow:
    ACK 47c4b1f52ab8d95d7deef83050bad49d1e3e5990
  furszy:
    light code review ACK 47c4b1f5
  aureleoules:
    ACK 47c4b1f52ab8d95d7deef83050bad49d1e3e5990

Tree-SHA512: d908dad00d1a5645eb865c4877cc0bae74b9cd3332a3641eb4a285431aef119f9fc78172d38b55c592168a73dae83242e6af3348815f7b37cbe2d448a3a58648

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