feat(llms): add FunctionTool for LLM function calling support

Add FunctionTool data classes and serialization support for LLM function calling feature. Includes Parameters schema handling with JSON serialization capabilities.

Author: phodal

core/src/main/kotlin/cc/unitmesh/devti/llms/call/FunctionTool.kt

@@ -0,0 +1,85 @@
+/*
+ * MIT License
+ *
+ * Copyright (c) 2025 https://github.com/aallam/openai-kotlin
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+package cc.unitmesh.devti.llms.call
+
+import kotlinx.serialization.KSerializer
+import kotlinx.serialization.SerialName
+import kotlinx.serialization.Serializable
+import kotlinx.serialization.descriptors.SerialDescriptor
+import kotlinx.serialization.encoding.Decoder
+import kotlinx.serialization.encoding.Encoder
+import kotlinx.serialization.json.*
+
+@JvmInline
+@Serializable
+public value class ToolType(public val value: String) {
+    public companion object {
+        public val Function: ToolType = ToolType("function")
+    }
+}
+
+@Serializable
+data class FunctionToolTool(
+    @SerialName("type") val type: ToolType,
+    @SerialName("function") val function: FunctionTool,
+)
+
+@Serializable
+public data class FunctionTool(
+    @SerialName("name") val name: String,
+    @SerialName("parameters") val parameters: Parameters? = null,
+    @SerialName("description") public val description: String? = null
+)
+
+@Serializable(with = Parameters.JsonDataSerializer::class)
+public data class Parameters(public val schema: JsonElement) {
+    public object JsonDataSerializer : KSerializer<Parameters> {
+        override val descriptor: SerialDescriptor = JsonElement.serializer().descriptor
+
+        override fun deserialize(decoder: Decoder): Parameters {
+            require(decoder is JsonDecoder) { "This decoder is not a JsonDecoder. Cannot deserialize `FunctionParameters`." }
+            return Parameters(decoder.decodeJsonElement())
+        }
+
+        override fun serialize(encoder: Encoder, value: Parameters) {
+            require(encoder is JsonEncoder) { "This encoder is not a JsonEncoder. Cannot serialize `FunctionParameters`." }
+            encoder.encodeJsonElement(value.schema)
+        }
+    }
+
+    public companion object {
+        public fun fromJsonString(json: String): Parameters = Parameters(Json.parseToJsonElement(json))
+
+        public fun buildJsonObject(block: JsonObjectBuilder.() -> Unit): Parameters {
+            val json = kotlinx.serialization.json.buildJsonObject(block)
+            return Parameters(json)
+        }
+
+        public val Empty: Parameters = buildJsonObject {
+            put("type", "object")
+            putJsonObject("properties") {}
+        }
+    }
+}