Refactor signing publications (#281)

Add a new function for signing publications

## Motivation and Context
In the [latest release
build](https://github.com/modelcontextprotocol/kotlin-sdk/actions/runs/17830036899/job/50692417727),
the artifact signing tasks were not included

## Breaking Changes
no

## Types of changes
- [x] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] Documentation update

## Checklist
- [x] I have read the [MCP
Documentation](https://modelcontextprotocol.io)
- [x] 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

Author: devcrocod

buildSrc/src/main/kotlin/mcp.publishing.gradle.kts

@@ -1,3 +1,5 @@
+import com.vanniktech.maven.publish.MavenPublishBaseExtension
+
 plugins {
     `maven-publish`
     id("com.vanniktech.maven.publish")
@@ -6,6 +8,7 @@ plugins {
 
 mavenPublishing {
     publishToMavenCentral(automaticRelease = true)
+    configureSigning(this)
 
     pom {
         name = project.name
@@ -42,13 +45,7 @@ mavenPublishing {
     }
 }
 
-publishing {
-    repositories {
-        mavenLocal()
-    }
-}
-
-signing {
+private fun Project.configureSigning(mavenPublishing: MavenPublishBaseExtension) {
     val gpgKeyName = "GPG_SIGNING_KEY"
     val gpgPassphraseName = "SIGNING_PASSPHRASE"
     val signingKey = providers.environmentVariable(gpgKeyName)
@@ -57,7 +54,7 @@ signing {
         .orElse(providers.gradleProperty(gpgPassphraseName))
 
     if (signingKey.isPresent) {
-        useInMemoryPgpKeys(signingKey.get(), signingPassphrase.get())
-        sign(publishing.publications)
+        mavenPublishing.signAllPublications()
+        signing.useInMemoryPgpKeys(signingKey.get(), signingPassphrase.get())
     }
 }