Merge bitcoin/bitcoin#34179: refactor: Enable transparent lookup for setBlockIndexCandidates to remove const_cast

3bd98b4508 refactor: use transparent comparator for setBlockIndexCandidates lookups (joaonevess)

Pull request description:

  ### Rationale

  This PR improves code safety by removing a `const_cast` in `src/validation.cpp`.

  Currently, `setBlockIndexCandidates` stores mutable `CBlockIndex*`. However, validation logic (like `CVerifyDB`) often holds `const CBlockIndex*`. Previously, checking for existence in the set required casting away constness. While currently benign, this bypasses compiler safety checks and could mask accidental modifications in future refactors.

  ### Description

  1.  **Enable Heterogeneous Lookup:** Added `using is_transparent = void;` to `CBlockIndexWorkComparator` in `src/node/blockstorage.h`. This allows the `std::set` to natively accept `const CBlockIndex*` for lookup (utilizing C++14 heterogeneous lookup).
  2.  **Remove Cast:** Removed the now unnecessary `const_cast<CBlockIndex*>` in `src/validation.cpp`, allowing the compiler to strictly enforce const-correctness.

  ### Notes

  - **Refactoring only:** No behavioral change.
  - **Verification:** `validation_tests` and `blockmanager_tests` pass.

ACKs for top commit:
  maflcko:
    review ACK 3bd98b4508 🚪
  frankomosh:
    ACK 3bd98b4508. Good use of transparent comparator to eliminate `const_cast` in this specific code path.
  sedited:
    ACK 3bd98b4508

Tree-SHA512: 0f76bdce2a54b759dfec99633afce1e95586e62f4057ecf1e82eed1a073eb8ecb2d659ccbf28a7a139f0aa09a30f058ac6966cafdfbf1f2ee878fa2d86b2c487

src/node/blockstorage.h

@@ -136,6 +136,7 @@ using BlockMap = std::unordered_map<uint256, CBlockIndex, BlockHasher>;
 
 struct CBlockIndexWorkComparator {
     bool operator()(const CBlockIndex* pa, const CBlockIndex* pb) const;
+    using is_transparent = void;
 };
 
 struct CBlockIndexHeightOnlyComparator {