feat(http/unstable): add isRetriableFetchError

[tomas-zijdemans]

Adds isRetriableFetchError, the missing predicate for retry()'s isRetriable hook. It recognizes transient fetch failures without substring-matching arbitrary error messages.

Implements the predicate half of #7296. The parser half is #7299.

Retriable cases

The predicate checks the thrown value and up to 8 objects in its cause chain, cycle-safe:

• a Response, or an Error with an integer status from 100 through 599, when the status is 408, 429, or 5xx
• an Error whose code is a known transient network code: ConnectionRefused or Timeout (Bun), or the errno-style ECONNREFUSED, ECONNRESET, ETIMEDOUT, ENOTFOUND, EAI_AGAIN (Bun and Node.js, including the cause of undici's fetch failed)
• a TimeoutError DOMException from APIs such as AbortSignal.timeout()
• a TypeError with an exact known failed-fetch message: fetch failed on Node.js and Deno 2.9+, one of the three browser strings, or the error sending request prefix from older Deno releases

AbortError stays false. Abort is caller intent, so retrying it is a bug.

Design notes

1. This uses 500 <= status < 600 instead of isServerErrorStatus. That helper rejects unregistered codes such as Cloudflare's 520-526. A 522 test guards this.
2. Modern Deno throws fetch failed, not error sending request ..., since fix(ext/fetch): reject transport failures with Node's "fetch failed" shape deno#35618. The older prefix stays because pre-2.9 Deno remains in the CI matrix.
3. Bun does not throw fetch failed yet (alignment is in flight in fetch: reject network errors as TypeError('fetch failed' / 'terminated') with the system error as cause oven-sh/bun#35988 and fetch: reject network errors as TypeError('fetch failed') with cause, errno codes, and a caller stack oven-sh/bun#35998). Bun 1.4 throws TypeErrors identified by code, with messages that cannot be exact-matched: the DNS failure embeds the hostname. Hence the code allowlist, verified against real Bun 1.4 fetch failures. Deterministic codes such as ERR_INVALID_URL stay excluded, and a test guards non-string codes.

Review notes

• Message matching is case-sensitive. That excludes deterministic Deno policy failures such as Fetch failed: Requests to port 6000 are blocked.
• A cross-runtime test closes an ephemeral local port, fetches it, and checks the real transport error. The test also runs under Node and Bun, the first http/ test registered there.
• Errors that store a Response on .response, including ky/got-style errors, are outside this API.
• The remaining test-bun failure (isGlob works with the input that includes large number of open brackets) is unrelated: a Bun 1.4.0 regression in node:worker_threads Worker construction that also reproduces on main.

Testing

deno task ok and deno task test:node pass, and deno task test:bun (Bun 1.4.0) passes this module's tests. The new module has 19 tests with 100% line and branch coverage.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.04%. Comparing base (ca58f94) to head (f5a3aa7).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7298   +/-   ##
=======================================
  Coverage   95.03%   95.04%           
=======================================
  Files         617      618    +1     
  Lines       51637    51698   +61     
  Branches     9359     9381   +22     
=======================================
+ Hits        49075    49136   +61     
  Misses       2021     2021           
  Partials      541      541           

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