Fix RegistryTool.tests type to match TestCase

[todo-mdx]

TypeScript Error: RegistryTool.tests type is incompatible with TestCase[] causing error in api.ts line 119.

RegistryTool.tests has: { name: string; input: unknown; expect: unknown }[]
TestCase expects: { name: string; input: unknown; expect: { status: 'success' | 'error'; ... } }[]

Location: src/mcp-registry.ts line 23

TDD Steps:

• RED: Run pnpm typecheck and observe error
• GREEN: Update RegistryTool interface to use TestCase type from testing convention
• REFACTOR: Ensure type exports are consistent across modules