fix: polish SEP-1577 sampling handling

[MukundaKatta]

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