chore(cli)!: replace node-fetch with undici

[X-Guardian]

Description

@cdktn/cli-core depended on the legacy node-fetch package and its proxy helper https-proxy-agent. This PR replaces that with the undici package — a modern, actively maintained fetch implementation with first-class proxy and mocking support — and removes node-fetch, @types/node-fetch, and https-proxy-agent.

Changes

• Fetch → undici. All three files now import { fetch } from "undici" instead of node-fetch. We use undici's exported fetch rather than the global fetch deliberately: it keeps the whole HTTP stack — code and test mocks — on a single undici instance. globalThis.fetch uses Node's internal undici, which is a different instance from the undici npm package, so mocking has to target the same one the code uses.

• Proxy → undici ProxyAgent via the dispatcher option. node-fetch's agent option (a Node http.Agent from https-proxy-agent) is replaced by an undici ProxyAgent built from the same http_proxy / HTTP_PROXY env vars and passed as the request dispatcher.

• Proxy support extended to package-manager.ts. Previously only registry-api.ts and prebuilt-providers.ts honoured a proxy; package-manager.ts (the PyPI / NuGet / Maven Central / GitHub / npm availability probes) never did. It now uses the same dispatcher, so all registry traffic respects a corporate proxy consistently.

• undici@8.6.0 dependency + Node floor. undici is added to @cdktn/cli-core. undici 8 requires Node ≥ 22.19.0, so both published packages (@cdktn/cli-core and cdktn-cli) declare "engines": { "node": ">=22.19.0" } — surfacing a clear install-time warning on older Node instead of an opaque runtime failure.

• Tests → undici MockAgent. prebuilt-providers.test.ts and package-manager.test.ts exercise undici's fetch, so their nock scopes are replaced with an undici MockAgent wired via setGlobalDispatcher.

Checklist

• I have updated the PR title to match CDKTN's style guide
• I have run the linter on my code locally
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation if applicable
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works if applicable
• New and existing unit tests pass locally with my changes