Merge bitcoindevkit/bdk#2174: fix(chain): prevent `merge_chains` from replacing the...

Merge bitcoindevkit/bdk#2174: fix(chain): prevent `merge_chains` from replacing the genesis block

be8e156115e21750158d44394341f1d41dfbf90f fix(chain): prevent `merge_chains` from replacing the genesis block (Elias Rohrer)

Pull request description:

  ### Description

  Other code paths (`disconnect_from`, `CheckPoint::insert`) already protect height 0 from modification, but `merge_chains` allowed an update chain with a different genesis hash to silently replace the wallet's existing genesis block. Return `CannotConnectError` when the update attempts to change the block hash at height 0.

  Co-Authored-By: HAL 9000
  ### Checklists

  #### All Submissions:

  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)

  #### Bugfixes:

  * [ ] This pull request breaks the existing API
  * [x] I've added tests to reproduce the issue which are now passing
  * [ ] I'm linking the issue being fixed by this PR

ACKs for top commit:
  ValuedMammal:
    ACK be8e156115e21750158d44394341f1d41dfbf90f
  evanlinjin:
    ACK be8e156115e21750158d44394341f1d41dfbf90f

Tree-SHA512: 743c95f281faf39f7f825693e684ad96a70b2994a6186721a68eacd0c3d4488092883e6dfe3b349e5241fc9a85a02dad222da7e932a50be5324df69d73cbb4a6