Merge bitcoindevkit/bdk#2053: Remove panic from stop gap scan loop

Merge bitcoindevkit/bdk#2053: Remove panic from stop gap scan loop

745f805068ce33ee830b497c28fd37eb4c1aafb1 fix: remove panic from stop gap scan loop (Matthew)

Pull request description:

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  ### Description

  This PR addresses a panic path in stop gap scan loop. Removes `expect` used to compute gap boundary and instead tracks `consecutive_unused` with `gap_limit = stop_gap.max(1)` to decide when to stop scanning.

  Spurred by trying to address https://github.com/bitcoindevkit/bdk_wallet/issues/30 for Esplora

  ### Notes to the reviewers

  Open to any and all feedback on this.

  Behavior is unchanged for typical cases, it just avoids a panic in the control flow.

  ### Changelog notice

  ```
  ### Fix:
  - Avoid a panic in the Esplora stop‑gap scan loop by tracking consecutive unused scripts to compute the gap boundary.
  ```

  ### Checklists

  #### All Submissions:

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

  #### New Features:

  * [ ] I've added tests for the new feature
  * [ ] I've added docs for the new feature

  #### Bugfixes:

  * [ ] This pull request breaks the existing API
  * [ ] 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:
  luisschwab:
    ACK 745f805068ce33ee830b497c28fd37eb4c1aafb1
  evanlinjin:
    ACK 745f805068ce33ee830b497c28fd37eb4c1aafb1

Tree-SHA512: 50b45955d7807996d871453dac61caf1ce25e85d31e261ebd670749b6370e92318d43070147aa67df5b4a904bd9b353b08711e043007c79a125cfd7d9a108da3