Merge bitcoindevkit/bdk#1443: fix(electrum): Fix `fetch_prev_txout`

af15ebba94f6d96a4d266fbbdee7c49150f80b96 fix(electrum): Fix `fetch_prev_txout` (valued mammal)

Pull request description:

  Previously we inserted every `TxOut` of a previous tx at the same outpoint, which is incorrect because an outpoint only points to a single `TxOut`. Now just get the `TxOut` corresponding to the txin prevout and insert it with its outpoint.

  ### Notes to the reviewers

  The bug in question was demonstrated in a discord comment https://discord.com/channels/753336465005608961/1239693193159639073/1239704153400414298 but I don't think we've opened an issue yet. Essentially, because of a mismatch between the outpoint and txout stored in TxGraph, we weren't summing the inputs correctly which caused `calculate_fee` to fail with `NegativeFee` error.

  fixes #1419

  ### 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:

  * [ ] I've added tests to reproduce the issue which are now passing

ACKs for top commit:
  LagginTimes:
    ACK af15ebba94f6d96a4d266fbbdee7c49150f80b96
  evanlinjin:
    ACK af15ebba94f6d96a4d266fbbdee7c49150f80b96

Tree-SHA512: c3a2c374069b0863076784856d90c7760b8f411e4881c3e42367015542b02ea6010c37745fb6dde95422af7222b7939ec51bc0fd1f63f816813c2159a17e08e5