Fix: parse Accept header q-values in wpsc_get_accept_header()

[thisismyurl]

Summary

Fixes #1045 — wpsc_get_accept_header() was using a naive str_contains() check that matched any JSON media type anywhere in the Accept string, ignoring RFC 7231 §5.3.2 quality values. The result: monitoring tools like New Relic Synthetics (and Datadog/Pingdom/UptimeRobot) that send valid HTML-preferring headers such as:

Accept: text/html,application/xhtml+xml,application/json;q=0.9,...

were misclassified as JSON requests. Two downstream effects:

1. wp_cache_serve_cache_file() refused to serve the cached file on every synthetic check.
2. A separate application/json cache bucket was populated on each check, triggering a fresh page build every time.

Changes

wp-cache-phase2.php

• Extracted a new wpsc_parse_accept_header( $raw_accept, $json_types ) helper. It parses q-values per RFC 7231 §5.3.2 and classifies the request as application/json only when a known JSON type has a strictly higher q-value than text/html. Ties resolve to text/html (safe default: serve the cached page).
• wpsc_get_accept_header() now delegates to this helper after the empty-header early-return. The static memoization and wpsc_accept_headers filter contract are unchanged. Callers are unaffected.

Edge cases handled:

• Malformed q-values (non-numeric, out-of-range) — treated as q=1.0, no warning or fatal.
• */* wildcard — provides effective html q-value only when text/html is not explicitly present.
• Whitespace around media-range tokens.

tests/php/WpscParseAcceptHeaderTest.php (new file)

14 unit tests covering the seven acceptance criteria from the issue brief plus additional edge cases (wildcard behaviour, custom types via the filter, typical browser headers, out-of-range q-values). The test exercises wpsc_parse_accept_header() directly — it has no WordPress dependencies, so no WP test scaffolding is required.

PHPUnit 12.5.28 — 14 / 14 (100%) — 0.003s

Note on test harness: the issue brief mentions a harness being stood up in #1051. I've added the test class here so the coverage lands with the fix rather than waiting. If #1051 introduces a shared bootstrap or test base class, this file is easy to migrate.

Acceptance criteria from the issue brief

• New Relic Synthetics header (text/html implicit q=1.0, application/json;q=0.9) → text/html
• Accept: application/json alone → application/json
• Accept: text/html,application/json (tie, both q=1.0) → text/html
• Accept: application/json,text/html;q=0.9 (json strictly higher) → application/json
• Accept: application/ld+json;q=1.0,text/html;q=0.8 (extended type via filter) → application/json
• Absent or empty Accept → text/html
• Malformed q-values → no warning or fatal, deterministic result
• wpsc_accept_headers filter types participate in q-value comparison
• Function remains free of late-loaded dependencies; static memoization preserved