Merge bitcoin/bitcoin#29668: prune, rpc: Check undo data when finding pruneheight

8789dc8f315a9d9ad7142d831bc9412f780248e7 doc: Add note to getblockfrompeer on missing undo data (Fabian Jahr)
4a1975008b602aeacdad9a74d1837a7455148074 rpc: Make pruneheight also reflect undo data presence (Fabian Jahr)
96b4facc912927305b06a233cb8b36e7e5964c08 refactor, blockstorage: Generalize GetFirstStoredBlock (Fabian Jahr)

Pull request description:

  The function `GetFirstStoredBlock()` helps us find the first block for which we have data. So far this function only looked for a block with `BLOCK_HAVE_DATA`. However, this doesn't mean that we also have the undo data of that block, and undo data might be required for what a user would like to do with those blocks. One example of how this might happen is if some blocks were fetched using the `getblockfrompeer` RPC. Blocks fetched from a peer will have data but no undo data.

  The first commit here allows `GetFirstStoredBlock()` to check for undo data as well by passing a parameter. This alone is useful for #29553 and I would use it there.

  In the second commit I am applying the undo check to the RPCs that report `pruneheight` to the user. I find this much more intuitive because I think the user expects to be able to do all operations on blocks up until the `pruneheight` but that is not the case if undo data is missing. I personally ran into this once before and now again when testing for assumeutxo when I had used `getblockfrompeer`. The following commit adds test coverage for this change of behavior.

  The last commit adds a note in the docs of `getblockfrompeer` that undo data will not be available.

ACKs for top commit:
  achow101:
    ACK 8789dc8f315a9d9ad7142d831bc9412f780248e7
  furszy:
    Code review ACK 8789dc8f315a9d9ad7142d831bc9412f780248e7.
  stickies-v:
    ACK 8789dc8f315a9d9ad7142d831bc9412f780248e7

Tree-SHA512: 90ae8bdd07a496ade579aa25240609c61c9ed173ad38d30533f6c631fe674e5a41727478ade69ca4b71a571ad94c9da4b33ebba6b5d8821109313c2de3bdfb3d

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