Spec conformance: meta support and spec updates

[aharvard]

This PR updates the Rust SDK to align more closely with the 2025-06-18 Model Context Protocol specification and adds first-class support for _meta.

Summary

• Protocol/version: set LATEST to 2025-06-18
• _meta support: added to text and image content, and embedded resources
• Annotated model: rename timestamp -> last_modified (schema lastModified)
• Prompt helpers: added function to create text messages with _meta
• Tests: kept existing Rust tests passing locally; Python-dependent tests are skipped due to local env

Key changes

• crates/rmcp/src/model.rs
  • Update LATEST constant to 2025-06-18
• crates/rmcp/src/model/content.rs
  • Add optional _meta to RawTextContent and RawImageContent
  • Comments clarifying _meta intent and examples
• crates/rmcp/src/model/prompt.rs
  • new_text_with_meta helper for convenience
  • Test updates to include _meta in RawImageContent case
• crates/rmcp/src/model/annotated.rs
  • Rename timestamp to last_modified and update accessors
• crates/rmcp/tests/test_message_schema.rs
  • Reviewed; remains compatible

Notes

• Local environment lacks Python, so Python-dependent tests were not executed; Rust tests pass.
• Happy to extend _meta to any additional surfaced interfaces if maintainers prefer a broader/narrower interpretation of where _meta is allowed in the June 18, 2025 spec.

Follow-ups (open to maintainer guidance)

• Confirm the precise set of interfaces that should expose _meta in the SDK surface area
• Add helpers for setting params._meta on requests (if desired)
• Tighten schema/tests once final interpretation is agreed

Thanks!