fix(client): resolve REST endpoint from discovery profile

[ynachiket]

The happy path client was ignoring the endpoint field from the /.well-known/ucp discovery response and instead hardcoding all API paths against the initial --server_url.

Per the UCP spec (Endpoint Resolution section):

"The endpoint field provides the base URL for API calls. OpenAPI
paths are appended to this endpoint to form the complete URL."

This works by coincidence when the merchant serves the API at its domain root (e.g. http://localhost:8182), but breaks when the merchant mounts the API under a path prefix (e.g.
https://merchant.com/buy/v1). In that case the client would hit https://merchant.com/checkout-sessions instead of the correct https://merchant.com/buy/v1/checkout-sessions, resulting in 403/404.

This commit:

• Adds endpoint_resolution.py with the resolve_rest_endpoint() helper
• Uses the resolved endpoint to recreate the httpx client base_url
• Updates exported curl URLs to reflect the resolved endpoint
• Falls back to --server_url if no endpoint is found in discovery
• Adds 9 unit tests covering path-prefixed, root-level, multi-transport, and missing/empty endpoint scenarios

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