Fix Base64{Url} incorrectly handling small destinations when inputs contain whitespace

[Copilot]

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)

Original prompt

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This section details on the original issue you should resolve

<issue_title>Base64{Url} incorrectly handles small destinations when inputs contain whitespace</issue_title>
<issue_description>```c#
byte[] input = Encoding.UTF8.GetBytes(" zA== ");
Console.WriteLine(Base64Url.DecodeFromUtf8(input, new byte[5])); // writes 1 byte
Console.WriteLine(Base64Url.DecodeFromUtf8(input, new byte[1])); // "Destination is too short."

```c#
_ = Base64Url.DecodeFromChars(new string(' ', 8), new byte[1]); // "Destination is too short."

Our logic in the core DecodeFrom workhorse isn't properly accounting for the possibility of skippable whitespace when calculating how much input we can process for a given destination size.</issue_description>

Comments on the Issue (you are @copilot in this section)

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

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Pull request overview

This pull request fixes a bug in Base64{Url}.DecodeFromUtf8 and DecodeFromChars where they incorrectly return DestinationTooSmall when the input contains whitespace and the destination is small but sufficient for the actual data.

Changes:

• Fixed the core decoder logic to fall through to whitespace-handling fallback when ignoreWhiteSpace is true at the DestinationTooSmallExit label
• Added comprehensive regression tests for the bug scenarios

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/libraries/System.Private.CoreLib/src/System/Buffers/Text/Base64Helper/Base64DecoderHelper.cs	Added fallthrough to InvalidDataExit when ignoreWhiteSpace is true at DestinationTooSmallExit, allowing whitespace to be stripped and destination size to be re-evaluated
src/libraries/System.Memory/tests/Base64Url/Base64UrlDecoderUnitTests.cs	Added three test methods covering: decoding with whitespace into small destination for UTF8 and chars, and verifying genuine DestinationTooSmall errors are still reported correctly