ci: add Codecov coverage upload and XML report generation (#579)

## 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
- [x] CI update

## Checklist
<!-- Go over all the following points, and put an `x` in all the boxes
that apply. -->
- [x] I have read the [MCP
Documentation](https://modelcontextprotocol.io)
- [x] My code follows the repository's style guidelines
- [x] New and existing tests pass locally
- [ ] I have added appropriate error handling
- [ ] I have added or updated documentation as needed

## Additional context
<!-- Add any other context, implementation notes, or design decisions
-->

.github/workflows/build.yml

@@ -36,7 +36,7 @@ jobs:
         include:
           - os: ubuntu-latest
             job-name: "Linux Tests"
-            gradle-tasks: "build ktlintCheck koverLog koverHtmlReport publishToMavenLocal -Pversion=1-SNAPSHOT"
+            gradle-tasks: "build ktlintCheck koverLog koverHtmlReport koverXmlReport publishToMavenLocal -Pversion=1-SNAPSHOT"
             max-workers: 3
 
           - os: windows-latest
@@ -107,6 +107,24 @@ jobs:
           report_paths: '**/test-results/**/TEST-*.xml'
           truncate_stack_traces: false
 
+      - name: Upload coverage reports to Codecov
+        if: matrix.os == 'ubuntu-latest'
+        uses: codecov/codecov-action@v5
+        with:
+          directory: ./build/reports/kover
+          files: report.xml
+
+      - name: Upload coverage reports to Codecov
+        if: ${{ !cancelled() && matrix.os == 'ubuntu-latest' }}
+        uses: codecov/codecov-action@v5
+        with:
+          directory: ./build/reports/kover
+          files: report.xml
+
+      - name: Upload test results to Codecov
+        if: ${{ !cancelled() && matrix.os == 'ubuntu-latest' }}
+        uses: codecov/test-results-action@v1
+
       - name: Archive Maven Local Repository
         if: ${{ success() && matrix.os == 'ubuntu-latest' }}
         run: |