fix: support edge/serverless runtimes via axios adapter fallback (#31)

[CedricConday]

Problem

The SDK throws in runtimes that don't ship Node's http module — Vercel Edge Functions, Cloudflare Workers, Next.js edge routes, Deno — because axios can't find a usable adapter (reported in #31):

AxiosError: There is no suitable adapter to dispatch the request since :
- adapter xhr is not supported by the environment
- adapter http is not available in the build

Fix

axios ≥ 1.7 accepts an ordered list of adapters and selects the first one supported by the current runtime. All requests already funnel through a single request() helper (src/utils/fetch.ts), so one change covers the whole SDK:

adapter: ["http", "xhr", "fetch"]

• Node → keeps using http (no behaviour change; all recorded-tape tests stay green)
• Browsers → keep using xhr
• Edge / serverless (no http/xhr) → transparently fall back to the universal fetch adapter

No new dependencies, no breaking changes, no major version needed. axios@^1.15.0 (already the declared dependency) ships the fetch adapter.

Tests

• Added src/utils/fetch.test.ts — a self-contained unit test (mocks axios, no network/API key) asserting the adapter fallback order.
• Full existing suite still passes (40 tape-replayed tests green; Node continues to select http).

Docs

Added a short Edge & serverless runtimes note to the README.

Fixes #31

🤖 Generated with Claude Code

https://claude.ai/code/session_01DbUHs855FyBt2J7cG2Qmhh

[Mantas97]

@CedricConday Clever solution for the problem 👍 Just one detail that needs to be addressed

[Mantas97]

Suggested change

	data: body && JSON.stringify(body),
	// Must be `undefined`, not `null`, when there is no body: workerd
	// (Cloudflare Workers) rejects a GET/HEAD Request that carries any
	// body property, even a null one. Node's http adapter tolerates it.
	data: body ? JSON.stringify(body) : undefined,

[Mantas97]

Worth adding one more test along with the fix:

describe("request body handling", () => {
    beforeEach(() => {
        axiosMock.mockClear();
    });

    // Regression: `data: body && JSON.stringify(body)` yielded `null` on
    // bodyless calls. Node's http adapter ignores that, but workerd throws
    // "Request with a GET or HEAD method cannot have a body", breaking every
    // read call on Cloudflare Workers once the fetch adapter is in play.
    it("omits `data` entirely when there is no body", async () => {
        await request("/api/subscribers", { method: "GET" }, config);

        const passedConfig = axiosMock.mock.calls[0][0] as { data: unknown };
        expect(passedConfig.data).toBeUndefined();
        expect(passedConfig.data).not.toBeNull();
    });

    it("still serializes a body when one is given", async () => {
        await request(
            "/api/subscribers",
            { method: "POST", body: { email: "test@example.com" } },
            config
        );

        const passedConfig = axiosMock.mock.calls[0][0] as { data: unknown };
        expect(passedConfig.data).toBe('{"email":"test@example.com"}');
    });
})