Validate timestamps when encoding JSON #994

Job	Run time
Test (Elixir 1.19.4 \| Erlang/OTP 28)	3m 18s
Conformance test (Elixir 1.16.3 \| Erlang/OTP 26)	11m 15s
Test (Elixir 1.17.3 \| Erlang/OTP 27)	3m 6s
Test (Elixir 1.18.1 \| Erlang/OTP 27)	2m 41s
Test (Elixir 1.16.3 \| Erlang/OTP 26)	3m 13s
	23m 33s

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