[M2] Implement diagnostics notification handling

[zacky1972]

Summary

Receive and store Lean diagnostics published by the language server.

Tasks

• Handle textDocument/publishDiagnostics notifications.
• Normalize diagnostics into Elixir structs or maps.
• Associate diagnostics with document URIs.
• Add tests for diagnostic payload parsing.

Acceptance criteria

• The session can return the latest diagnostics for an opened document.
• Diagnostic severity, range, and message are preserved.