ci: switch from coveralls to codecov for coverage reporting

[shubham-shinde-442]

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

Notes to the reviewers

Changelog notice

All Submissions:

• I followed the contribution guidelines

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.35%. Comparing base (8dd0209) to head (86287ae).
⚠️ Report is 1540 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2144      +/-   ##
==========================================
- Coverage   77.36%   77.35%   -0.02%     
==========================================
  Files          35       28       -7     
  Lines        9035     5466    -3569     
  Branches        0      237     +237     
==========================================
- Hits         6990     4228    -2762     
+ Misses       2045     1172     -873     
- Partials        0       66      +66     

Flag	Coverage Δ
rust	77.35% <ø> (?)

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[notmandatory]

ACK 86287ae

Confirmed using the same version of codecov GitHub action as bdk_wallet.
https://github.com/bitcoindevkit/bdk_wallet/blob/fca652370c466c79659afc6ae57c5015d4007b18/.github/workflows/code_coverage.yml#L40

[Toyin5]

Shouldn't the pipeline fail if error?

[notmandatory]

Good question, I don't think we need to fail, and we don't fail for the bdk_wallet crate either. But worth reconsidering if we begin to see a problem with new PRs not having adequate coverage.
https://github.com/bitcoindevkit/bdk_wallet/blob/fca652370c466c79659afc6ae57c5015d4007b18/.github/workflows/code_coverage.yml#L46