fix: prefer native fetch over node-fetch to eliminate url.parse() deprecation warning

[lukasjuhas]

Problem

Node.js 18+ users see this deprecation warning when using the SDK:

(node:12345) [DEP0169] DeprecationWarning: `url.parse()` behavior is not 
standardized and prone to errors that have security implications. 
Use the WHATWG URL API instead.

This warning originates from node-fetch v2.x, which internally uses the deprecated url.parse() API.

Solution

Update safeFetch.ts to check for native fetch availability before attempting to load node-fetch. Since Node.js 18+ includes native fetch, this eliminates the need for node-fetch entirely in modern environments.

Changes

1. src/utils/safeFetch.ts: Check for native fetch first, only fall back to node-fetch when native fetch is unavailable
2. src/__tests__/jest.setup.js: Delete global.fetch in tests so existing node-fetch mocks continue to work
3. src/utils/__tests__/safeFetch.test.ts: New tests to verify the native fetch preference behavior

Behavior by Environment

Environment	Fetch Used	Deprecation Warning
Node 18+	Native	❌ None
Node 14/16	node-fetch	⚠️ Expected
Edge/Browser	Native	❌ None
Tests	Mocked node-fetch	❌ None

Test Results

All 326 tests pass (excluding the expected ClientInitializeResponseConsistency failure for non-Statsig employees).

Backward Compatibility

• ✅ No breaking changes to the public API
• ✅ Existing tests continue to work with node-fetch mocks
• ✅ Falls back to node-fetch for older Node.js versions (14, 16)
• ✅ Works in edge runtimes and browsers that have native fetch