Replace Coveralls with Codecov for coverage reporting

[GideonBature]

Description

This PR migrates our code coverage reporting from Coveralls to Codecov while maintaining all existing functionality.

Notes to the reviewers

Changelog notice

Changes

• Updated workflow name: Changed from "Code Coverage" to reflect Codecov integration
• Replaced coverage service: Switched from coverallsapp/github-action to codecov/codecov-action@v4
• Enhanced permissions: Added necessary permissions for Codecov OIDC authentication:
  • contents: read
  • pull-requests: write
  • id-token: write
• Improved configuration: Added Codecov-specific parameters:
  • use_oidc: true for secure tokenless authentication
  • fail_ci_if_error: false to prevent CI failures on upload issues
  • flags: rust for better organization
  • name: codecov-bdk-wallet for clear identification

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran just p before pushing

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

Closes #320

[codecov]

Welcome to Codecov 🎉

Once you merge this PR into your default branch, you're all set! Codecov will compare coverage reports and display results in all future pull requests.

Thanks for integrating Codecov - We've got you covered ☂️

[notmandatory]

ACK 36f98df

@GideonBature good work!

Confirmed codecov/codecov-action@5a1091511ad55cbe89839c7260b706298ca349f7 is tagged version 5.5.1

Also verified report correctly generated for @GideonBature's fork of bdk_wallet:
https://coveralls.io/jobs/173295208

[notmandatory]

The published codecov report for bdk_wallet is here:
https://app.codecov.io/github/bitcoindevkit/bdk_wallet