feat: AG-UI as in-process KMP API with Ktor client/server transports

Model the AG-UI protocol as an in-process API of Java-style interfaces and
classes, mirroring the official AG-UI Java SDK, and build it as a Kotlin
Multiplatform library with two Ktor transport modules.

Modules:
- api: BaseEvent sealed hierarchy (all AG-UI event types), messages, state,
  tool, context, RunAgentInput/Parameters, Agent/AgentSubscriber/
  EventDispatcher, AguiJson (kotlinx.serialization, "type" discriminator),
  plus a SerializersModule registering every event subtype polymorphically.
- client: HttpAgent over Ktor SSE (POST RunAgentInput -> stream BaseEvent),
  implementing the Agent interface.
- server: Application.agui()/Route.aguiAgent() exposing an Agent behind a
  POST + SSE endpoint via SSEServerContent, plus a runnable sample.

Tests: api serialization round-trips, client SSE decode against a real
test-host server, server endpoint integration.

gradle/wrapper/gradle-wrapper.properties

@@ -0,0 +1,7 @@
+distributionBase=GRADLE_USER_HOME
+distributionPath=wrapper/dists
+distributionUrl=https\://services.gradle.org/distributions/gradle-8.14.3-bin.zip
+networkTimeout=10000
+validateDistributionUrl=true
+zipStoreBase=GRADLE_USER_HOME
+zipStorePath=wrapper/dists