Allow samples to run against local builds using mavenLocal()

[MarkusBillharz]

This PR introduces minimal changes to allow contributors to run the sample projects against local builds of the SDK, without breaking the independence of the samples.

Resolves #196

Changes

• Add mavenLocal() to the repositories block in the sample projects so they can resolve locally published artifacts.
• Add a short note to the contributing guidelines explaining how to publish locally for testing using ./gradlew publishToMavenLocal
• Increase the Kotlin version of the mcp-server-sample to get it to build (ABI error).
• Ensure the sample projects are included in the PR checks to verify they build.

This avoids directly linking the samples to the main project (as discussed in #119 (review)), while making it easier to test local changes during development.

To test it, I fake version bumped it on my clone to check that it is actually working as intended:
https://github.com/MarkusBillharz/kotlin-sdk/actions/runs/16523835710/job/46731929589

Types of changes

• Non-breaking changes
• Documentation update

Feedback welcome!