fix(http-client): throw on non-2xx responses so MCP tools surface errors correctly

[fuleinist]

Summary

Fixes #66

Before: response.ok was never checked, so server errors (500, 503, etc.) returned as successful tool results with the error text as the output. Example: /api/refine-ui returning "Anthropic experiencing high load" as HTTP 200 would appear as a successful refinement to the MCP client — the client could not distinguish an outage from a real refinement.

After: createMethod now throws an Error with the HTTP status and server error body when response.ok is false. MCP clients receive a proper error they can surface to the user.

Changes

• src/utils/http-client.ts — after fetch(), check response.ok. If false, read the response body as text and throw Error(HTTP {status} {statusText}: {errorText}). If the body can't be read, fall back to just the status text.
• src/utils/http-client.test.ts — 3 new regression tests covering:
  • 500 with JSON error body → full error message in thrown Error
  • 503 with no body → status-only thrown Error
  • 401 with JSON error body → full error message in thrown Error

Test results

PASS src/utils/http-client.test.ts
  http-client
    √ should use production URL in production environment (1 ms)
  http-client error handling
    √ throws on non-2xx response with status and error message (25 ms)
    √ throws on non-2xx response when response has no body (2 ms)
    √ throws on 401 with the error body from server (2 ms)
Tests: 4 passed, 4 total