fix(cache-control): reject invalid cache control times

[lprnmns]

cacheControl() and staticCache() convert string times with @lukeed/ms and then only check typeof time === 'number'. ms() returns undefined for a string it can't parse, so the time becomes NaN, passes the check and ends up in the header:

reply.cacheControl('max-age', 'forever') // Cache-Control: max-age=NaN
reply.staticCache('two days')            // Cache-Control: public, max-age=NaN, immutable
reply.maxAge(-1)                         // Cache-Control: max-age=-1

Caches treat these directives as invalid, and nothing tells the developer.

A shared toSeconds() helper now converts the time and asserts that it is a finite, non-negative number. maxAge() and stale() go through cacheControl(), so they are covered too. Valid inputs keep producing the same headers.

Tests cover an unparsable string, NaN, Infinity and a negative number. npm test passes, including 100% coverage and the type tests, and so does eslint.

Checklist

• run npm run test
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin and the Code of conduct