Merge bitcoindevkit/bdk#2144: ci: switch from coveralls to codecov for coverage reporting

2a42f2517036325588c7681b5efc90b3c725a119 ci(coverage): use `codecov` instead of `coveralls` (Shubham Shinde)

Pull request description:

  closes #2093

  ### Description

  Migrates code coverage reporting from Coveralls to Codecov.

  - **Replaced coverage service**: Switched from `coverallsapp/github-action` to `codecov/codecov-action@v5.5.2` (pinned commit SHA for security)
  - **Added permissions**: Added `contents: read` and `pull-requests: write` to enable Codecov PR comments
  - **Codecov configuration**: Added `flags: rust` and `name: codecov-bdk` for better organization

  ### Changelog notice

  ```
  ### Changed
  - chore(ci): switch from coveralls to codecov
  ```
  #### All Submissions:

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

ACKs for top commit:
  oleonardolima:
    self-ACK 2a42f2517036325588c7681b5efc90b3c725a119
  notmandatory:
    ACK 2a42f2517036325588c7681b5efc90b3c725a119

Tree-SHA512: 53dd898fefb973fc4a884fbfb34335d5f03992d30c676841b66db8dcc5a4d3a8e5178d5886cb91308df873f25fe6703457dc27aa5b45451f828bfcded612cace