fix: allow deserializing notifications without params field

[DaleSeo]

Fixes #703

Motivation and Context

The JSON-RPC 2.0 spec makes params optional in notifications, but Notification::deserialize was using Proxy, which requires that field to be present. This led to users getting an untagged enum deserialization error when calling serde_json::from_slice::<JsonRpcMessage>(&body) with the bare JsonRpcMessage type on notifications that didn't include params, like notifications/initialized. The typed aliases, such as ClientJsonRpcMessage, weren't affected because they go through NotificationNoParam for notifications without parameters.

To fix this, we switched Notification::deserialize to use the existing ProxyOptionalParam. Now, when params is missing, it treats it as an empty object {} before deserializing into R.

How Has This Been Tested?

Added a regression test

Breaking Changes

None. This is a pure bug fix. Previously failing parses now succeed, and all existing behaviour for notifications with params is unchanged.

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

Additional context