refactor(mcp): tighten connection result types

[rekram1-node]

Summary

• use explicit connected and unavailable tags for MCP connection outcomes
• use the existing local and remote MCP config variants without casts
• require clients and tool definitions on connected creation results
• remove unchecked type assertions, undefined-client placeholders, and non-null assertions
• validate tool arguments at the SDK boundary instead of casting them

Validation

• bun test test/mcp/lifecycle.test.ts
• bun typecheck

Stacked on: #31549
Relates to: #28567