docs: clarify structuredContent is not LLM "structured outputs"

Add a note to the Structured Content section of the server tools spec clarifying that `structuredContent` is server-produced result data, unrelated to LLM provider "structured outputs" (schema-constrained model generation). The terms are routinely conflated. Applied to the draft, 2025-11-25, and 2025-06-18 revisions.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>