Merge bitcoin/bitcoin#34124: validation: make CCoinsView a pure virtual interface

8783cc8056 refactor: inline `CCoinsViewBacked` implementation (Lőrinc)
86296f276d coins: make `CCoinsView` methods pure virtual (Lőrinc)
b637566c8d coins: add explicit `CoinsViewEmpty` noop backend (Lőrinc)
90c635c01c fuzz: keep backend assertions aligned to active backend (Lőrinc)
a9f92e3497 refactor: normalize CCoinsView whitespace and signatures (Lőrinc)
38a99f3344 scripted-diff: normalize `CCoinsView` naming (Lőrinc)
06172ef0d5 refactor: rename `hashBlock` to `m_block_hash` to avoid shadowing (Lőrinc)

Pull request description:

  ### Problem
  `CCoinsView` is the coins view interface, but historically it also provided built-in no-op behavior:

  * It provided default no-op implementations (returning `std::nullopt`, `uint256()`, `false`, or `nullptr`) instead of being pure virtual.
  * Callers could instantiate a bare `CCoinsView` and get silent no-op behavior.
  * Mixing the interface definition with a built-in dummy implementation blurred the abstraction boundary.

  ### Context
  This is part of the ongoing coins caching cleanup in #34280.

  ### Fix
  This PR separates the interface from no-op behavior and makes `CCoinsView` pure virtual in incremental steps:
  * Add `CoinsViewEmpty` as an explicit no-op coins view for tests, benchmarks, and temporary backends.
  * Replace direct bare-`CCoinsView` test and dummy instantiations with `CoinsViewEmpty`.
  * Make all `CCoinsView` methods pure virtual (`PeekCoin`, `GetCoin`, `HaveCoin`, `GetBestBlock`, `GetHeadBlocks`, `BatchWrite`, `Cursor`, `EstimateSize`).
  * Remove the legacy default implementations from `coins.cpp`.
  * Update fuzz and dummy backends to use `CoinsViewEmpty` explicitly.

ACKs for top commit:
  w0xlt:
    reACK 8783cc8056
  ryanofsky:
    Code review ACK 8783cc8056. Just updated comments and variable name since last review. The fuzz test code is much clearer now IMO
  andrewtoth-exo:
    ACK 8783cc8056
  ajtowns:
    ACK 8783cc8056

Tree-SHA512: cfc831578aa309788c1b5dafbfecca3de388cc4215534c3f3df24f90d7770ed37b1fd7aa134df91d611d0a1ca75929accb98d5ed7df7b52851c259e04f08e4a3

src/node/psbt.cpp

@@ -116,8 +116,7 @@ PSBTAnalysis AnalyzePSBT(PartiallySignedTransaction psbtx)
 
         // Estimate the size
         CMutableTransaction mtx(*psbtx.tx);
-        CCoinsView view_dummy;
-        CCoinsViewCache view(&view_dummy);
+        CCoinsViewCache view{&CoinsViewEmpty::Get()};
         bool success = true;
 
         for (unsigned int i = 0; i < psbtx.tx->vin.size(); ++i) {