SEP-1319: Decouple Request Payloads + _meta erasure fix (modelcontextprotocol#1284) + "breaking" arbitrary params fix (modelcontextprotocol#1692)

* Ensure _meta is available on all requests (modelcontextprotocol#1284)

* PR feedback

* Address Notifications, take a stab at handling the intent behind the breaking change

* PR Feedback: `any` instead of `unknown` with union

* Apply suggestions from code review

Co-authored-by: Jonathan Hefner <[email protected]>

* re-run generation

* Actually make the intended breaking change

See: modelcontextprotocol#1319 (comment)

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Co-authored-by: Jonathan Hefner <[email protected]>

Author: ccreighton-apptio