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/libs.versions.toml

@@ -0,0 +1,25 @@
+[versions]
+kotlin = "2.2.0"
+kotlinx-coroutines = "1.10.2"
+kotlinx-serialization = "1.8.1"
+ktor = "3.1.3"
+
+[libraries]
+kotlinx-coroutines-core = { module = "org.jetbrains.kotlinx:kotlinx-coroutines-core", version.ref = "kotlinx-coroutines" }
+kotlinx-coroutines-test = { module = "org.jetbrains.kotlinx:kotlinx-coroutines-test", version.ref = "kotlinx-coroutines" }
+kotlinx-serialization-json = { module = "org.jetbrains.kotlinx:kotlinx-serialization-json", version.ref = "kotlinx-serialization" }
+
+ktor-server-core = { module = "io.ktor:ktor-server-core", version.ref = "ktor" }
+ktor-server-sse = { module = "io.ktor:ktor-server-sse", version.ref = "ktor" }
+ktor-server-netty = { module = "io.ktor:ktor-server-netty", version.ref = "ktor" }
+ktor-server-content-negotiation = { module = "io.ktor:ktor-server-content-negotiation", version.ref = "ktor" }
+ktor-server-test-host = { module = "io.ktor:ktor-server-test-host", version.ref = "ktor" }
+
+ktor-client-core = { module = "io.ktor:ktor-client-core", version.ref = "ktor" }
+ktor-client-cio = { module = "io.ktor:ktor-client-cio", version.ref = "ktor" }
+ktor-client-content-negotiation = { module = "io.ktor:ktor-client-content-negotiation", version.ref = "ktor" }
+ktor-serialization-kotlinx-json = { module = "io.ktor:ktor-serialization-kotlinx-json", version.ref = "ktor" }
+
+[plugins]
+kotlin-multiplatform = { id = "org.jetbrains.kotlin.multiplatform", version.ref = "kotlin" }
+kotlin-serialization = { id = "org.jetbrains.kotlin.plugin.serialization", version.ref = "kotlin" }