fix: enforce must-revalidate and proxy-revalidate over max-stale and stale-if-error

[jeswr]

What

The cache interceptor's isStale() granted the request max-stale grace window without consulting the stored response's directives, and handleResult() applied the stale-if-error threshold unconditionally.

RFC 9111 §5.2.2.2: a cache MUST NOT use a stale response with must-revalidate to satisfy any request without successful validation — this overrides the request's max-stale (see the note in §5.2.1.2) and also excludes the response from stale-if-error (RFC 5861 §4). proxy-revalidate has the same semantics for shared caches — the interceptor's default type — and was parsed but never enforced anywhere.

Fixes #5508

Repro

Store 200 Cache-Control: max-age=1, must-revalidate (+ ETag), wait past staleness, then request with Cache-Control: max-stale=600:

• Before (undici 8.7.0 / current main, Node 22): served straight from cache, zero origin contact.
• After: conditional request (if-none-match/if-modified-since) sent to the origin; the cached response is only reused after successful validation.

Same for stale-if-error: a stale must-revalidate response was previously served when revalidation returned a 5xx within the stale-if-error window; now the error is propagated.

Changes

• lib/interceptor/cache.js: new forbidsServingStale(result, cacheType) helper; isStale() skips the max-stale grace window and handleResult() skips the stale-if-error threshold when the stored response has must-revalidate (or proxy-revalidate on a shared cache). proxy-revalidate correctly remains inert for type: 'private' caches.
• test/interceptors/cache.js: three new tests (must-revalidate vs max-stale, proxy-revalidate shared vs private, must-revalidate excluded from stale-if-error), all failing before the fix and passing after.

Testing

• New tests fail on main (stale served with 0 revalidation requests / 200 instead of 500) and pass with the fix.
• Full cache interceptor suite (test/interceptors/cache*.js, test/cache-interceptor/*.js): 124 tests, 124 pass, 0 fail (121 baseline + 3 new).
• mnot cache-tests conformance runner (node test/cache-interceptor/cache-tests.mjs): identical to main's baseline — 283 total, 220 passed, 0 failed, 58 failed-optional, 5 setup-failed. (The suite has no test for the max-stale + must-revalidate combination, so no skip-list entries could be un-skipped; the skipped stale-close-must-revalidate test depends on the connection-close revalidation path, which is out of scope here.)

Notes

• This change is agent-assisted (Claude Fable 5) working with @jeswr.
• Touches the same handleResult() region as the sibling PR fix: honor request cache-control max-age=0 in cache interceptor #5510 (fix/cache-request-max-age-zero), but both are self-contained against main and merge-independent.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.44%. Comparing base (cb4c2f1) to head (c391280).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5511   +/-   ##
=======================================
  Coverage   93.44%   93.44%           
=======================================
  Files         110      110           
  Lines       37328    37345   +17     
=======================================
+ Hits        34881    34898   +17     
  Misses       2447     2447           

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