Merge bitcoin/bitcoin#26533: prune: scan and unlink already pruned block files on startup

3141eab9c6 test: add functional test for ScanAndUnlinkAlreadyPrunedFiles (Andrew Toth)
e252909e56 test: add unit test for ScanAndUnlinkAlreadyPrunedFiles (Andrew Toth)
77557dda4a prune: scan and unlink already pruned block files on startup (Andrew Toth)

Pull request description:

  There are a few cases where we can mark a block and undo file as pruned in our block index, but not actually remove the files from disk.
  1. If we call `FindFilesToPrune` or `FindFilesToPruneManual` and crash before `UnlinkPrunedFiles`.
  2. If on Windows there is an open file handle to the file somewhere else when calling `fs::remove` in `UnlinkPrunedFiles` (https://en.cppreference.com/w/cpp/filesystem/remove, https://learn.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-deletefilew#remarks). This could be from another process, or if we are calling `ReadBlockFromDisk`/`ReadRawBlockFromDisk` without having a lock on `cs_main` (which has been allowed since ccd8ef65f9).

  This PR mitigates this by scanning all pruned block files on startup after `LoadBlockIndexDB` and unlinking them again.

ACKs for top commit:
  achow101:
    ACK 3141eab9c6
  pablomartin4btc:
    re-ACK with added functional test 3141eab9c6.
  furszy:
    Code review ACK 3141eab9
  theStack:
    Code-review ACK 3141eab9c6

Tree-SHA512: 6c73bc57838ad1b7e5d441af3c4d6bf4c61c4382e2b86485e57fbb74a61240710c0ceeceb8b4834e610ecfa3175c6955c81ea4b2285fee11ca6383f472979d8d

src/validation.cpp

@@ -4323,6 +4323,8 @@ bool ChainstateManager::LoadBlockIndex()
         bool ret = m_blockman.LoadBlockIndexDB(GetConsensus());
         if (!ret) return false;
 
+        m_blockman.ScanAndUnlinkAlreadyPrunedFiles();
+
         std::vector<CBlockIndex*> vSortedByHeight{m_blockman.GetAllBlockIndices()};
         std::sort(vSortedByHeight.begin(), vSortedByHeight.end(),
                   CBlockIndexHeightOnlyComparator());