ci: add Codecov coverage upload and XML report generation

[kpavlov]

Add Codecov coverage upload and XML report generation

• Add koverXmlReport to Gradle tasks in Ubuntu workflow
• Configure Codecov action to upload coverage reports

Reports will be available on https://app.codecov.io/gh/modelcontextprotocol/kotlin-sdk

Motivation and Context

To provide better test coverage visibility

How Has This Been Tested?

CI

Breaking Changes

No

Types of changes

• CI update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

[codecov-commenter]

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.

ℹ️ You can also turn on project coverage checks and project coverage reporting on Pull Request comment

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

[zarechneva-a]

LGTM, thank you

[devcrocod]

Why do we need this?

We shouldn’t introduce a third-party AI tool without a clear and explicit need!

Especially considering that, instead of codecov, we could add the claude plugin or qodana, which would provide similar functionality. And likely in a more reliable way than an external AI tool. And that’s only if we actually need this in the first place

[kpavlov]

@devcrocod, just so you know, this isn’t an AI tool. It’s simply a viewer for the generated coverage XML report that Kover creates.

I don’t think there’s anything wrong with adding a reporting tool. What I’m more worried about is the lack of reporting and visibility.

Qodana is a bit slow, and I’m happy to consider switching to another tool that offers better UX.