[setup-r] retry transient r-hub.io request failures

[nbenn]

determineVersion() resolves the R version with a single GET to api.r-hub.io through getJSON(), with no retry, so a transient timeout or server error fails the whole job — e.g. Error: Request timeout: /rversions/resolve/release/linux-ubuntu-24.04/x86_64.

This wraps the fetch in getJSON() in a small retry loop: 4 attempts, exponential backoff (1s / 5s / 25s), and a per-attempt 15s timeout via AbortSignal.timeout() so a stalled connection aborts and the next attempt can proceed.

Only transient failures are retried — thrown network/timeout errors and 5xx / 429 / 408 responses. A 404, or any other stable response, still returns undefined immediately, and once the retries are exhausted getJSON() returns undefined the same way — it never throws. So both callers' existing behaviour is preserved: determineVersion() still does its arm64 → intel macOS fallback and raises Failed to resolve R version …, and getReleaseVersion() still degrades to an empty version string. Both failure paths — a non-retryable status, and exhausted retries — are logged with core.debug, matching the rest of the file. I put the retry in getJSON() rather than at the one resolve call site so that getReleaseVersion() — the other r-hub caller — gets the same resilience; happy to narrow it to just the resolve GET if you'd prefer.

I verified the control flow locally against a stubbed fetch: immediate success → one call, no retry; transient error / 5xx / 429 → retries then recovers; 404 → no retry, returns undefined; persistent failure → returns undefined after 4 attempts so the caller falls back as before. The setup-r action has no unit-test harness, so I didn't add a test here — glad to wire one up if that would be welcome.

dist/index.js rebuilt with ncc.

Fixes #1086

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.33%. Comparing base (d3c5be5) to head (e318527).

Additional details and impacted files

@@            Coverage Diff             @@
##           v2-branch    #1088   +/-   ##
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  Coverage      83.33%   83.33%           
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  Files              3        3           
  Lines             12       12           
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  Hits              10       10           
  Misses             2        2           

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