Add configurable MCP toolkit support

[Chel-Tbl]

Summary

Closes #82.

Adds a configurable MCP toolkit so users can connect Toolkit.dev to any hosted MCP server from the toolkit selector.

Changes

• Add an MCP toolkit with url and optional JSON headers parameters.
• Discover hosted MCP tools dynamically with experimental_createMCPClient.
• Prefix discovered MCP tools as mcp_* so they can be passed through the existing chat tool pipeline.
• Close MCP clients after stream completion or stream errors.
• Add a generic tool invocation renderer for dynamic tools that do not have static client components.
• Tighten the existing MCP export route typing so the new empty/dynamic toolkit shape keeps typechecking.

Testing

• pnpm exec prettier --check "src/app/api/chat/route.ts" "src/app/api/mcp/[server]/[transport]/route.ts" "src/app/(general)/_components/chat/messages/message-tool.tsx" "src/toolkits/toolkits/mcp/**/*.ts" "src/toolkits/toolkits/mcp/**/*.tsx" "src/toolkits/toolkits/shared.ts" "src/toolkits/toolkits/client.ts" "src/toolkits/toolkits/server.ts"
• pnpm exec tsc --noEmit --pretty false
• SKIP_ENV_VALIDATION=1 pnpm run build-pure