Merge bitcoin/bitcoin#32950: validation: remove BLOCK_FAILED_CHILD

fb3e1bf9c9 test: check LoadBlockIndex correctly recomputes invalidity flags (stratospher)
29740c06ac validation: remove BLOCK_FAILED_MASK (stratospher)
b5b2956bda validation: reset BLOCK_FAILED_CHILD to BLOCK_FAILED_VALID when loading from disk (stratospher)
37bc207852 validation: stop using BLOCK_FAILED_CHILD (stratospher)
120c631e16 refactor: use clearer variables in InvalidateBlock() (stratospher)
18f11695c7 validation: don't update BLOCK_FAILED_VALID to BLOCK_FAILED_CHILD in InvalidateBlock (stratospher)

Pull request description:

  Fixes https://github.com/bitcoin/bitcoin/issues/32173

  even though we have a distinction between `BLOCK_FAILED_VALID` and `BLOCK_FAILED_CHILD` in the codebase,
  we don't use it for anything. Whenever we check for BlockStatus, we use `BLOCK_FAILED_MASK` which encompasses both of them.

  Since there is no functional difference between `BLOCK_FAILED_VALID` and `BLOCK_FAILED_CHILD` and it's added
  code complexity to correctly categorise them (ex: https://github.com/bitcoin/bitcoin/pull/31405#discussion_r1914366243, https://github.com/bitcoin/bitcoin/pull/16856#issuecomment-565506585), we could just remove it.

  Looking for conceptual feedback on whether it's better to improve handling of `BLOCK_FAILED_CHILD` in the codebase or remove `BLOCK_FAILED_CHILD`.

  Of less relevance, but it would also fix a `reconsiderblock` crash that could happen in the situation mentioned in https://github.com/bitcoin/bitcoin/issues/32173#issuecomment-2767030982

  Similar attempt in the past in https://github.com/bitcoin/bitcoin/pull/16856#issuecomment-568073859

ACKs for top commit:
  stickies-v:
    re-ACK fb3e1bf9c9
  alexanderwiederin:
    ACK fb3e1bf9c9
  mzumsande:
    re-ACK fb3e1bf9c9

Tree-SHA512: e97b739885c40a8c021966438e9767cc02bc183056236d6a8c64f6819347ae70c0fbcd71cc2528917560d9f4fd56aed45faf1b6c75d98de7b08b621693a97fbc

src/rpc/mining.cpp

@@ -742,7 +742,7 @@ static RPCHelpMan getblocktemplate()
             if (pindex) {
                 if (pindex->IsValid(BLOCK_VALID_SCRIPTS))
                     return "duplicate";
-                if (pindex->nStatus & BLOCK_FAILED_MASK)
+                if (pindex->nStatus & BLOCK_FAILED_VALID)
                     return "duplicate-invalid";
                 return "duplicate-inconclusive";
             }