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>

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
- * than two messages or no tool_use / tool_result blocks are involved.
+ * On the first violation throws [IllegalArgumentException]. No-op when no
+ * 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,
+        )
+    }
 }