SEP-1577 sampling follow-up: invariants, validator messages, conformance

[devcrocod]

Polish items deferred from #718

Tasks

• Enforce non-empty content in SamplingMessage and CreateMessageResult via init { require(content.isNotEmpty()) }.
  kotlin-sdk-core/.../types/sampling.kt — refs r3142638028, r3142638034

• Fix validateSamplingMessages KDoc — it claims no-op for <2 messages, but a single message with tool_result throws.
  kotlin-sdk-server/.../SamplingValidation.kt — ref r3142638012

• **Better error when tool_result is missing — currently throws "ids do not match" with an empty set; add an explicit require(toolResultIds.isNotEmpty()) first.
  kotlin-sdk-server/.../SamplingValidation.kt — ref r3142638015

• test_sampling conformance tool returns TextContent(text=…, type=…) instead of the sampled text — extract TextContent.text, handle non-text blocks explicitly.
  conformance-test/.../ConformanceTools.kt — ref r3142638007

related to #409