fix(mcp): report the bound HTTP port

[Iams4kura]

Summary

• Fixes: Starting the MCP server with --transport http --port 0 successfully binds an ephemeral port, but the startup URL is logged as http://localhost:0/mcp and cannot be used to connect.
• Root cause: The listening callback interpolates the requested port argument instead of reading the actual TCP address assigned by Node after the server starts listening.

Regression evidence

• Before: npx -y pnpm@10 --filter @upstash/context7-mcp exec vitest run test/integration.test.ts -t 'reports the bound port when port zero requests an ephemeral port' exited 1

• After: npx -y pnpm@10 --filter @upstash/context7-mcp exec vitest run test/integration.test.ts -t 'reports the bound port when port zero requests an ephemeral port' exited 0

Verification

• npx -y pnpm@10 --filter @upstash/context7-mcp test
• npx -y pnpm@10 build
• npx -y pnpm@10 lint:check
• npx -y pnpm@10 format:check
• npx -y pnpm@10 typecheck

Scope

• 3 files changed, +26 / -7 lines