[Feat]: Migrate HTTP transport from superagent to fetch for edge-runtime compatibility

[liuqiaowei512]

Feature Request Is New

• I have verified that the requested feature does not already exist or has not already been requested.

Description of the feature

Problem

The client can't run on edge runtimes (Cloudflare Workers, Vercel Edge, Deno Deploy) because:

• All requests go through superagent, which relies on node:http/node:https (Node) or XHR (browser) — neither exists in these runtimes; they only provide fetch.
• src/easypost.js builds the default User-Agent from os.platform()/os.release()/process.versions.node at module-eval time, so even importing the package throws.

Proposal

Replace superagent with the standard fetch API (built into Node ≥18, all browsers, and every edge runtime).

The coupling is well-contained: services only call _request() and consume response.body/status/headers, so a fetch-based _request returning the same shape leaves services/, models/, and errors/ untouched. Sketch:

• URL/query via URL + URLSearchParams, Basic auth via an Authorization header, timeout via AbortSignal.timeout()
• Static User-Agent string (no os/process)
• Breaking: superagentMiddleware/requestMiddleware/useProxy expose superagent objects — replace with an injectable fetch option (covers proxying and mocking); requestHooks/responseHooks already use plain objects and survive as-is
• Drops the superagent dependency and shrinks the install

Benefits

• Runs everywhere: Node ≥18, browsers, Cloudflare Workers, Vercel Edge, Deno, Bun — one transport, zero polyfills.
• Fewer dependencies: drops superagent and its transitive chain (formidable, hexoid, …), shrinking install size and supply-chain surface.
• Eliminates a known bug class: [Bug]: Cannot instantiate new EasyPostClient in Nextjs 13 #439 (Next.js build failure via superagent → formidable → hexoid) was a symptom of this chain; fetch removes the class of problem rather than patching around it.
• No bundler workarounds: users stop needing webpack overrides to build in Next.js and similar frameworks.
• Future-proof: fetch is the platform standard; superagent-era conveniences (JSON parsing, query strings, timeouts) are each a few lines over fetch.

Happy to contribute a PR if there's interest.

[Justintime50]

I'm a fan of moving on this; however, it would necessitate a major version bump and thus require a bit of planning on our part with other work to be done on this lib. I'll get this in our backlog and keep you posted here on our progress. Thanks for the suggestion!

[Justintime50]

Working on this now; however, you mentioned wanting to run these in edge compatible environments. Is node available in these locations you want to run this library? It is intended as a backend dependency which requires node, running without (even if switching to fetch will lead to failure due to the numerous node-specific call sites of other functions in this library. The README does also state it's intended for node only (early version of this lib, released years ago, were potentially compatible in more "browser-like" envs, but that no longer holds true).

That added context will help. I do still intend to make the swap from superagent to fetch but the proposal to move away from os in the issue summary gave me pause.