fix: reject unsafe-integer maxAge in stringifySetCookie

[ilovesugarr]

Summary

Fix stringifySetCookie emitting invalid Max-Age=1e+21 exponential notation headers for integers >= 1e21.

Motivation

Number.isInteger(1e21) evaluates to true, but converting 1e21 to a string produces "1e+21", violating RFC 6265 §5.2.2 grammar (max-age-value = 1*DIGIT).

Implementation

Use Number.isSafeInteger instead of Number.isInteger in stringifySetCookie.

Testing

Added ["unsafe-integer", 1e21] test case in src/stringify-set-cookie.spec.ts.

[blakeembrey]

Closing in favor of #286