Description

Base64{Url}.DecodeFromUtf8 and DecodeFromChars incorrectly return DestinationTooSmall when input contains whitespace and destination is small but sufficient for actual data.

byte[] input = Encoding.UTF8.GetBytes("  zA==  ");
Base64Url.DecodeFromUtf8(input, new byte[5]); // ✓ writes 1 byte
Base64Url.DecodeFromUtf8(input, new byte[1]); // ✗ was: "Destination is too short"

Base64Url.DecodeFromChars(new string(' ', 8), new byte[1]); // ✗ was: "Destination is too short"

Root cause: DecodeFrom calculates maxSrcLength from total source length (including whitespace). When destLength < decodedLength - 2, it jumps to DestinationTooSmallExit before the whitespace-handling fallback can strip whitespace and re-evaluate.

Fix: At DestinationTooSmallExit, when ignoreWhiteSpace is true, fall through to InvalidDataFallback which properly handles whitespace and determines if the actual content fits.

• Base64DecoderHelper.cs: Added fallthrough to InvalidDataExit when ignoreWhiteSpace is true
• Base64UrlDecoderUnitTests.cs: Added tests for leading/trailing whitespace scenarios that reproduce the issue
• Base64UrlDecoderUnitTests.cs: Added tests for embedded whitespace scenarios (e.g., "z A==") that trigger the DecodeWithWhiteSpaceBlockwiseWrapper path
• Base64DecoderUnitTests.cs: Added similar tests for the Base64 decoder since both Base64 and Base64Url share the same underlying implementation
• Added test cases for whitespace-only input with empty destination buffer (size 0)

• Fixes Base64{Url} incorrectly handles small destinations when inputs contain whitespace #123222

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