fix: tighten setRequestHandler types for Client and Server

[mattzcarey]

Summary

• 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

Follows up on #1446 with minimal code changes (+12/-4 lines).

Test plan

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

🤖 Generated with Claude Code

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Latest commit: 05cfcf6

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commit: 05cfcf6

[mattzcarey]

Superseded by #1463 which uses a cleaner generic type parameter approach instead of @ts-expect-error