fix(fetch): clone request on retry to support requests with body

[lucasros98]

Summary

Closes #78.

fetchRetry was passing the same Request object to fetchJSON on every retry attempt. Once a Request body has been consumed by a fetch call, the same Request object cannot be used again — the runtime throws:

TypeError: Cannot construct a Request with a Request object that has already been used.

This affected any POST/PUT request with a body when the first attempt returned a retryable status (5xx). GET requests without a body were unaffected, which is why the existing retry tests did not catch it.

Fix

Call `request.clone()` before each fetch attempt so the original `Request` remains unconsumed and the next retry can construct a fresh fetch from it.

-    const result = await fetchJSON<TOk, TErr>(request);
+    const result = await fetchJSON<TOk, TErr>(request.clone());

Test plan

Added a regression test (`fetchRetry - should retry POST request with body without consuming it`) that:

1. Mocks a POST endpoint that returns 500 on the first attempt and 200 on the second
2. Sends a `Request` with `method: POST` and a JSON body
3. Asserts that the retry succeeds

This test fails on `main` with the exact stack trace from #78:

at fetch (https://jsr.io/@hongminhee/deno-mock-fetch/0.3.2/mod.ts:56:17)
at fetchJSON (src/fetch.ts:63:26)
at attemptFetch (src/fetch.ts:33:26)
at attemptFetch (src/fetch.ts:46:12)

…and passes with this change. All existing tests continue to pass (89 total).

• `deno test` — 89 passed
• `deno lint` — clean
• `deno fmt --check` — clean