feat(client): refactor AgentikAgent to manage its own HttpClient

- `AgentikAgent` now accepts `engineFactory` and an optional `token` to create an internal `HttpClient`, handling all configuration (JSON, Bearer).
- Removed `applyAgentikDefaults` and replaced it with `agentikHttpClient` for `HttpClient` creation with consistent settings.
- Updated `Agent` to implement `AutoCloseable`, ensuring proper resource closure with `agent.close()`.
- Adjusted tests, docs, and examples to align with the new `AgentikAgent` API.

server/src/commonTest/kotlin/pw/binom/agentik/server/BearerTokenTest.kt

@@ -57,6 +57,7 @@ class BearerTokenTest {
         override suspend fun getConversation(id: String): Conversation? = null
         override suspend fun deleteConversation(id: String): Boolean = false
         override suspend fun getConversations(offset: Int, limit: Int): List<Conversation> = emptyList()
+        override fun close() {}
     }
 
     private suspend fun startServer(token: String?): Pair<EmbeddedServer<*, *>, Int> {