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