feat: Add init result instructions to client

[tymondesigns]

Hi there, I noticed that the instructions property that already exists on the InitializeResult class is not set on the Client.

This is useful to be able to set the system message within a prompt.

This can be used by clients to improve the LLM’s understanding of available tools, resources, etc. It can be thought of like a “hint” to the model. For example, this information MAY be added to the system prompt.

Equivalent PR for the TS client - modelcontextprotocol/typescript-sdk#133

References

• https://github.com/modelcontextprotocol/modelcontextprotocol/blob/main/schema/2024-11-05/schema.ts#L170-L175
• https://modelcontextprotocol.io/specification/2025-06-18/schema#initializeresult