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-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)
     // ============================================================================