feat: VTEX client and add_to_cart action support

[paulobernardoaf]

What

Adds an add_to_cart WebSocket message type that lets browser clients add or update items in a VTEX shopping cart. The implementation introduces a new pkg/vtex package containing an HTTP client for the VTEX Checkout API, wired into the existing WebSocket handler pipeline.

New package — pkg/vtex

• IClient interface with a single method AddOrUpdateCartItem(ctx, vtexAccount, orderFormID, itemID, seller) for testability
• Client implementation that fetches the current order form (GET), then either adds the item or increments its quantity (POST)
• gomock-generated mock (MockIClient) for handler-level tests

WebSocket integration

• New add_to_cart case in ParsePayload dispatch
• AddToCart handler validates the payload synchronously, then runs the VTEX API call in a goroutine (non-blocking for the read loop)
• Sends cart_updated on success or cart_error on failure back to the client

Hardening

• 30 s HTTP client timeout on the VTEX client to prevent goroutine leaks from slow/unresponsive upstream
• context.WithTimeout (30 s) wrapping the goroutine's VTEX call, enabling cancellation and bounding goroutine lifetime
• SSRF protection: vtexAccount and orderFormID are validated against ^[a-zA-Z0-9][a-zA-Z0-9-]*$ before URL interpolation, blocking fragment (#), slash, dot, and query-string injection attacks

Files changed

File	Change
pkg/vtex/client.go	New — VTEX Checkout API client
pkg/vtex/client_mock.go	New — gomock mock
pkg/vtex/client_test.go	New — httptest-based tests (add, update, errors, SSRF validation)
pkg/websocket/application.go	Added VTEXClient vtex.IClient to App and NewApp
pkg/websocket/client.go	AddToCart handler + ParsePayload case
pkg/websocket/client_test.go	8 new test cases (happy path, not registered, feature disabled, missing fields, VTEX error, ParsePayload integration)
api/main.go	Wiring: creates vtex.NewClient(), passes to NewApp

Why

Enables the webchat widget to add products to a VTEX shopping cart directly from the conversation, supporting e-commerce use cases where a chatbot recommends products and the user can add them without leaving the chat.