Improve proxy server testability

[kmcginnes]

The proxy server package (graph-explorer-proxy-server) has most of its logic in a single node-server.ts file that is difficult to test in isolation. Route handlers, middleware, server creation, and configuration resolution are all interleaved in module-level code.

This task tracks the work to decompose the proxy server into focused, independently testable modules:

• Extract createApp() — Express app with routes and middleware, testable via supertest
• Extract createServer() — HTTP/HTTPS server creation decision
• Extract resolveServerConfig() — Port selection, HTTPS detection, certificate paths, base URL
• Add global test setup with credential safety mocks (prevent real AWS credentials or HTTP requests in tests)

Remaining work was split into separate issues:

• Add tests for SPARQL/Gremlin query cancellation on client disconnect #1658
• Add integration tests for proxy server orchestrator #1659

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