MINOR: Improve efficiency of Uuid.randomUuid
#20788
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Avoid constructing a Base64 string inside the rejection loop by checking the high 6 bits of the most significant 64 bits directly. This preserves the existing guarantee that the Base64-URL encoded form does not start with a hyphen while reducing allocation and work in the hot path. UUIDs equal to 0 or 1 remain impossible due to the version-4 bit pattern.
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Avoid constructing a Base64 string inside the rejection loop by checking the high 6 bits of the most significant 64 bits directly. This preserves the existing guarantee that the Base64-URL encoded form does not start with a hyphen while reducing allocation and work in the hot path.
UUIDs equal to 0 or 1 remain impossible due to the version-4 bit pattern.