Build NPI Registry provider lookup module

Create a TypeScript module that queries the NPPES NPI Registry API to search and retrieve healthcare provider records.

Function signature:

lookupProvider(query: { name?: string, npi?: string, state?: string }): Promise<Provider[]>

The Provider interface should normalize the raw NPI response into: { id: string, name: string, credential: string, address: string, city: string, state: string, specialty: string, phone: string }.

Requirements:

1. Call https://npiregistry.cms.hhs.gov/api/?version=2.1 with the provided query params
2. Handle pagination (API returns max 200 per page; implement limit at 50 for now)
3. Return empty array on 404 or no results
4. Throw a typed NpiApiError on 5xx or malformed JSON
5. Never return providers with deactivated status

Write unit tests with 4 fixture cases: found by name, found by NPI, not found, API error. Use vi.mock to avoid real HTTP calls.