guard long cast of numeric jwt claim in extractJwtPayload

[netliomax25-code]

extractJwtPayload renders a numeric JWT claim via static_cast(value.number_value()), but that cast is undefined when the claim is non-finite or outside long's range. A token whose logged number claim is e.g. 1e19 (> LONG_MAX) trips it; under UBSan: "1e+19 is outside the range of representable values of type 'long'". The number comes straight from the client JWT. Guard the narrowing the way JsonStruct::getInteger already does and render the raw value otherwise.

[google-oss-prow]

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[google-oss-prow]

Hi @netliomax25-code. Thanks for your PR.

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