fix: send stored last-modified as if-modified-since when revalidating

[jeswr]

Fixes #5506

What

When revalidating a stale cached response, the cache interceptor built the conditional request with 'if-modified-since': new Date(result.cachedAt).toUTCString() — the local wall-clock time the response was inserted into the cache — on both the synchronous revalidation path and the stale-while-revalidate background path in lib/interceptor/cache.js.

RFC 9110 §13.1.3 says a cache should generate If-Modified-Since from the stored response's Last-Modified field value, falling back to the Date field value, and only then the local time the response was cached.

Concrete failure modes of the old behavior:

1. Exact-match origins never return 304. nginx's default is if_modified_since exact; since the local insertion time never equals the resource's Last-Modified, every Last-Modified-only revalidation (no ETag) against such origins gets a full 200, silently losing the 304 bandwidth saving.
2. Clock skew can serve stale data. If the client clock runs ahead of the origin, a resource modified between Last-Modified and (client-time) cachedAt is masked: the origin compares its mtime against a too-late date and answers 304.

Repro (before → after)

Origin responding Cache-Control: max-age=1 + Last-Modified: Sat, 09 Oct 2010 14:28:02 GMT, two requests >1s apart through a cache-composed dispatcher, logging the revalidation request's if-modified-since:

• Before (main): If-Modified-Since: <cache insertion wall-clock time> (e.g. Wed, 02 Jul 2026 ... GMT) — does not match the stored Last-Modified.
• After (this PR): If-Modified-Since: Sat, 09 Oct 2010 14:28:02 GMT — the stored value, verbatim.

Changes

• Add a getIfModifiedSinceValue(result) helper implementing the RFC 9110 §13.1.3 hierarchy (stored last-modified → stored date → cachedAt) and use it at both revalidation call sites. If-None-Match behavior is unchanged.
• Tests in test/interceptors/cache-revalidate-stale.js: verbatim Last-Modified on the synchronous revalidation path, on the stale-while-revalidate background path, and the Date-header fallback when the stored response has no Last-Modified. All three fail on main and pass with this change.

Tests

• node --test over the cache interceptor test files (test/interceptors/cache*.js, test/cache-interceptor/*): 124 tests / 13 suites, 124 pass, 0 fail (baseline on main: 121/121 + the 3 new tests).
• mnot cache-tests conformance runner (node test/cache-interceptor/cache-tests.mjs): Total 283 — Passed 222, Failed 0, Failed-optional 57, Setup failed 4 (equal or better than the main baseline of 220 / 0 / 58 / 5; the conformance suite has no If-Modified-Since-generation test, so no skip-list entries could be un-skipped by this change).

Notes

This change is agent-assisted (Claude Fable 5) working with @jeswr. It is part of a series of self-contained cache-interceptor conformance fixes against main; sibling PRs touching adjacent logic in the same interceptor: #5510 (fix/cache-request-max-age-zero) and #5511 (fix/cache-must-revalidate-max-stale).

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Codecov Report

❌ Patch coverage is 90.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.45%. Comparing base (cb4c2f1) to head (efe48e3).

Files with missing lines	Patch %	Lines
lib/interceptor/cache.js	90.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5512   +/-   ##
=======================================
  Coverage   93.44%   93.45%           
=======================================
  Files         110      110           
  Lines       37328    37346   +18     
=======================================
+ Hits        34881    34901   +20     
+ Misses       2447     2445    -2     

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