Implement SEP-1613: JSON Schema 2020-12 as Default Dialect

[felixweinberger]

This is a tracking issue for implementation of SEP-1613.

Summary

This SEP establishes JSON Schema 2020-12 as the default dialect for embedded schemas within MCP messages (tool inputSchema/outputSchema and elicitation requestedSchema fields). The TypeScript SDK needs to ensure all schema generation and validation uses JSON Schema 2020-12 by default, handle schemas that explicitly declare alternative dialects via the $schema field, and update documentation to reflect the standardized default version. This resolves validation failures between implementations assuming different versions.

Related Issues & PRs

• Implementation PRs: #1030, #1040, #914, #1012
• Related PRs: n/a
• Related Issues: n/a