refactor: add jsonrpc field to base Notification class

[felixweinberger]

Summary

Adds the jsonrpc: Literal["2.0"] = "2.0" field to the base Notification class, aligning with the TypeScript SDK where notification types extend JSONRPCNotification.

Motivation and Context

Issue #1729 identified that notification types like ProgressNotification inherit from Notification, not JSONRPCNotification, meaning they lack the jsonrpc: "2.0" field.

This PR addresses the Notifications part of that issue. The Results part is intentionally not changed because:

• Results need an id field that comes from the request
• Handlers don't know the request ID when returning a result
• The session layer must wrap results with the request ID
• The TypeScript SDK also keeps Results as payloads (extending Result, not JSONRPCResultResponse)

Partially fixes #1729

How Has This Been Tested?

• All 1100 tests pass
• pyright and ruff pass

Breaking Changes

None. The jsonrpc field has a default value, so existing code continues to work.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed