fix: tighten setRequestHandler types for Client and Server

[mattzcarey]

Summary

• Add ReceiveRequestMethod generic parameter to Protocol class
• Client.setRequestHandler() now only accepts server-to-client methods (sampling/createMessage, elicitation/create, roots/list)
• Server.setRequestHandler() now only accepts client-to-server methods (tools/call, prompts/get, resources/read, etc.)

This prevents accidentally registering handlers for wrong method types, following up on #1446.

The approach uses a generic type parameter on Protocol rather than @ts-expect-error comments, making it a clean TypeScript solution.

Test plan

• All tests pass (485/485)
• Type checking passes

🤖 Generated with Claude Code

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Latest commit: a886553

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commit: a886553