fix: reject unsafe-integer maxAge in stringifySetCookie

[spokodev]

stringifySetCookie guards maxAge with Number.isInteger, but that accepts integers >= 1e21, which String() renders in exponential notation. The result is a Max-Age that breaks the RFC 6265 §5.2.2 grammar (max-age-value = 1*DIGIT):

stringifySetCookie({ name: "foo", value: "bar", maxAge: 1e21 })
// => "foo=bar; Max-Age=1e+21"   (invalid; no compliant parser accepts it)

The guard exists precisely to guarantee a valid header (it already rejects Infinity/NaN/non-integers), so letting these values through defeats its purpose. It also corrupts a parse -> serialize round-trip: Max-Age=<22 digits> is a valid header a server may send, parseSetCookie reads it as 1e21, and re-serializing yields the broken output above.

Switching to Number.isSafeInteger keeps the emitted value a plain integer (safe integers are <= 2^53, well below the exponential threshold) and still accepts every realistic Max-Age. All existing cases (0, -1, 1000, and the Infinity/3.14/non-number throw cases) are unaffected.

Adds a regression test.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (4898ba2) to head (64f9cd1).
⚠️ Report is 40 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #286   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines          160       193   +33     
  Branches        69        83   +14     
=========================================
+ Hits           160       193   +33     

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