* feat: add _meta to content blocks and embedded resources; update schemas
* feat: set default protocol version; add _meta to content blocks/resources; update schemas
* chore: format content.rs via rustfmt
* chore(protocol): keep LATEST at 2025-03-26 per review until full 2025-06-18 compliance
* feat(prompt): add constructors with optional meta for image and resource
- Keep text helper; meta is currently ignored for text until schema supports it.
* refactor(prompt): simplify constructors so meta is optional; remove duplicate non-meta variants
* fix: modify code comment about version
* refactor(prompt): rename meta parameters in new_resource function for clarity
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