Run PHPStan at level 5 [ignore_release]

[snake14]

Description

This plugin had no phpstan.neon, so the umbrella skipped the check and it has never been analysed. The configuration matches the one the rest of the fleet uses; level 5 is what the comparably sized plugins already run at, and the plugin is clean at that level.

Six findings, which turned out to be three causes — all fixed in their own commit.

getRegexMatchEntry() and getNumEntriesToCache() are annotated @return string but both return (int)…. That one wrong annotation accounted for three of the six: the two return-type findings, and the int - non-falsy-string on the subtraction in WarmDeviceDetectorCache, which reads $numEntriesToCache from that method.

writeToCache() has taken a single parameter since 0c205c38, but every call site — one in the command and twelve in the tests — still passed a second empty array. PHP discards extra arguments to a userland function, so nothing broke; the calls now match the signature.

CachedEntry::__construct() left $clientHints untyped, so its type was inherited from DeviceDetector::__construct(..., ?ClientHints $clientHints = null). The class actually takes an array of headers and converts it with ClientHints::factory(), and all five call sites pass arrays. Declaring it array states the real contract and resolves both remaining findings.

No version bump or changelog entry: the substance is static-analysis setup.

Issue No

No GitHub issue.

Steps to Replicate the Issue

1. Note that the umbrella passes skip-phpstan for this plugin because there is no config to run.
2. Expected result: static analysis runs, as it does for the other migrated plugins.
3. Actual result: the check is skipped.

Checklist

• [✔] Tested locally or on demo2/demo3?
• [NA] New test case added/updated?
• [NA] Are all newly added texts included via translation?
• [NA] Are text sanitized properly? (Eg use of v-text v/s v-html for vue)
• [NA] Version bumped?
• [✔] I have understood, reviewed, and tested all AI outputs before use
• [✔] All AI instructions respect security, IP, and privacy rules
• [NA] Documentation updated?