feat(data-weaver): implement generic fetch nodes

[nick-nlb]

Description

This PR extracts a reusable fetchNodes client helper for querying the Data Commons V2 /v2/node endpoint and add unit test coverage for the dc_api client.

The proximate reason for this refactor and the generic function (rather than a single direct fetchChildPlaces) is that upcoming functionality to fetch GeoJSON for the choropleths will use this. More generally, a fetchNodes function could prove to be useful for other purposes later.

Testing

• Added unit tests in dc_api.test.ts covering missing API key errors, /v2/node request structure, and child place parsing.
• Verified test suite.
• Verified lint and type checks with pnpm lint.
• Verified build and build runtime

Note: this does not fix formatting, as a separate open PR does this.

[gemini-code-assist]

Code Review

This pull request refactors the Data Commons API client by extracting a reusable fetchNodes function from fetchChildPlaces and introduces a comprehensive test suite in dc_api.test.ts. The review feedback recommends improving the test setup by using Vitest's built-in vi.stubEnv and vi.unstubAllEnvs utilities instead of manually mutating process.env. Additionally, it is recommended to use vi.spyOn(globalThis, 'fetch') rather than directly overwriting globalThis.fetch to prevent global scope pollution and potential test flakiness.