Merge bitcoindevkit/bdk#1202: fix(chain): filter coinbase tx not in best chain

991cb77b6fbeedbf52d1bd9aa6b3d680f8269969 fix(chain): filter coinbase tx not in best chain (Wei Chen)

Pull request description:

  ### Description

  Fixes #1144.
  Coinbase transactions cannot exist in the mempool and be unconfirmed. `TxGraph::try_get_chain_position` should always return `None` for coinbase transactions not anchored in best chain.

  ### Checklists

  #### All Submissions:

  * [x] I've signed all my commits
  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)
  * [x] I ran `cargo fmt` and `cargo clippy` before committing

  #### Bugfixes:

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

ACKs for top commit:
  notmandatory:
    ACK 991cb77b6fbeedbf52d1bd9aa6b3d680f8269969
  danielabrozzoni:
    ACK 991cb77b6fbeedbf52d1bd9aa6b3d680f8269969

Tree-SHA512: 9e06d8404708eee050c96807a876a470303f4983666c82c56c17d97c2d4b72784e75271279fd393c53a6a967a352aea1ef2762da71ac4bb58f7a0c2f05354948