fix: polish SEP-1577 sampling handling (#765)

## Summary
- reject empty sampling content in SamplingMessage and
CreateMessageResult constructors
- clarify sampling validation docs and emit an explicit error when
tool_use is not followed by tool_result
- return sampled text from the conformance test_sampling tool instead of
content object strings

Fixes #730

## Testing
- ./gradlew
-Dorg.gradle.java.installations.paths="/Users/ubl/Documents/New
project/oss-contributions/.jdks/jdk-21.0.11+10/Contents/Home"
:kotlin-sdk-core:jvmTest :kotlin-sdk-server:jvmTest
:conformance-test:compileKotlin

Co-authored-by: Pavel Gorgulov <devcrocod@gmail.com>

conformance-test/src/main/kotlin/io/modelcontextprotocol/kotlin/sdk/conformance/ConformanceTools.kt

@@ -179,7 +179,13 @@ fun Server.registerConformanceTools() {
                 ),
             ),
         )
-        CallToolResult(listOf(TextContent(result.content.joinToString("\n") { it.toString() })))
+        val sampledText = result.content.joinToString("\n") { content ->
+            when (content) {
+                is TextContent -> content.text
+                else -> "Non-text sampling content: ${content::class.simpleName}"
+            }
+        }
+        CallToolResult(listOf(TextContent(sampledText)))
     }
 
     // 9. Elicitation

kotlin-sdk-core/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/types/sampling.kt

@@ -91,6 +91,10 @@ public data class SamplingMessage(
     @SerialName("_meta")
     val meta: JsonObject? = null,
 ) {
+    init {
+        require(content.isNotEmpty()) { "content must contain at least one block" }
+    }
+
     /**
      * Convenience constructor for a single-block message. Wraps [content] in a
      * singleton list so call sites can write `SamplingMessage(Role.User, TextContent("hi"))`
@@ -273,6 +277,10 @@ public data class CreateMessageResult(
     @SerialName("_meta")
     override val meta: JsonObject? = null,
 ) : ClientResult {
+    init {
+        require(content.isNotEmpty()) { "content must contain at least one block" }
+    }
+
     /**
      * Convenience constructor for a single-block response. Wraps [content] in a
      * singleton list so call sites can write

kotlin-sdk-core/src/commonTest/kotlin/io/modelcontextprotocol/kotlin/sdk/types/SamplingTest.kt

@@ -302,6 +302,13 @@ class SamplingTest {
         (m.content[0] as TextContent).text shouldBe "hi"
     }
 
+    @Test
+    fun `SamplingMessage rejects empty content`() {
+        assertFailsWith<IllegalArgumentException> {
+            SamplingMessage(role = Role.User, content = emptyList())
+        }
+    }
+
     @Test
     fun `SamplingMessage single-element content serialises as single object`() {
         val m = SamplingMessage(role = Role.User, content = listOf(TextContent("hi")))
@@ -408,6 +415,17 @@ class SamplingTest {
         (decoded.content[0] as TextContent).text shouldBe "hi"
     }
 
+    @Test
+    fun `CreateMessageResult rejects empty content`() {
+        assertFailsWith<IllegalArgumentException> {
+            CreateMessageResult(
+                role = Role.Assistant,
+                content = emptyList(),
+                model = "test-model",
+            )
+        }
+    }
+
     // ============================================================================
     // SamplingContentSerializer (single-or-array wire heuristic)
     // ============================================================================

kotlin-sdk-server/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/server/SamplingValidation.kt

@@ -20,8 +20,8 @@ import io.modelcontextprotocol.kotlin.sdk.types.ToolUseContent
  * 3. If the previous message contains `tool_use` blocks, the last message's
  *    `tool_result` ids MUST form exactly the same set.
  *
- * On the first violation throws [IllegalArgumentException]. No-op when there are fewer
+ * On the first violation throws [IllegalArgumentException]. No-op when no
- * than two messages or no tool_use / tool_result blocks are involved.
+ * tool_use / tool_result blocks are involved.
  */
 internal fun validateSamplingMessages(messages: List<SamplingMessage>) {
     if (messages.isEmpty()) return
@@ -44,6 +44,9 @@ internal fun validateSamplingMessages(messages: List<SamplingMessage>) {
     if (hasPreviousToolUse) {
         val toolUseIds = previous.filterIsInstance<ToolUseContent>().map { it.id }.toSet()
         val toolResultIds = last.filterIsInstance<ToolResultContent>().map { it.toolUseId }.toSet()
+        require(toolResultIds.isNotEmpty()) {
+            "tool_use blocks from previous message must be followed by matching tool_result blocks"
+        }
         require(toolUseIds == toolResultIds) {
             "ids of tool_result blocks and tool_use blocks from previous message do not match"
         }

kotlin-sdk-server/src/jvmTest/kotlin/io/modelcontextprotocol/kotlin/sdk/server/SamplingTest.kt

@@ -8,6 +8,7 @@ import io.modelcontextprotocol.kotlin.sdk.types.ToolUseContent
 import kotlinx.serialization.json.JsonObject
 import org.junit.jupiter.api.assertDoesNotThrow
 import kotlin.test.Test
+import kotlin.test.assertEquals
 import kotlin.test.assertFailsWith
 
 /**
@@ -83,4 +84,21 @@ class SamplingTest {
             )
         }
     }
+
+    @Test
+    fun `validate tool_use requires explicit tool_result in last message`() {
+        val error = assertFailsWith<IllegalArgumentException> {
+            validateSamplingMessages(
+                listOf(
+                    SamplingMessage(Role.Assistant, toolUse("c1")),
+                    SamplingMessage(Role.User, TextContent("missing result")),
+                ),
+            )
+        }
+
+        assertEquals(
+            "tool_use blocks from previous message must be followed by matching tool_result blocks",
+            error.message,
+        )
+    }
 }