Fix Base64 decoding edge cases with small destinations and whitespace

[MihaZupan]

Fixes #123222
Replaces #123258

From InvalidDataFallback we'll call back into DecodeFrom to try and process more data.
But if the destination is too small and the next block of Base64 contains whitespace, it won't be able to make progress. Use the block-wise fallback in those cases.

Tests in #123151 now pass when including this change.

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[Copilot]

Pull request overview

This PR fixes edge cases in Base64 decoding when dealing with small destination buffers and embedded whitespace in the input. The core issue was that the InvalidDataFallback method would get stuck when trying to decode data with whitespace interspersed in Base64 blocks when the destination buffer was too small to hold the complete decoded output.

Changes:

• Modified the loop exit condition in InvalidDataFallback to be more explicit about which statuses terminate the loop
• Added logic to detect when DecodeFrom returns DestinationTooSmall but makes no progress (localConsumed == 0), falling back to block-wise decoding in those cases
• Added comprehensive test coverage for the edge case with whitespace-heavy input and small destination buffers
• Enhanced existing test assertions to verify Base64.DecodeFromUtf8 behavior is consistent with Convert.TryFromBase64String

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/libraries/System.Private.CoreLib/src/System/Buffers/Text/Base64Helper/Base64DecoderHelper.cs	Fixed the InvalidDataFallback loop to properly handle DestinationTooSmall status by continuing to consume whitespace when progress is made, or falling back to block-wise decoding when stuck
src/libraries/System.Memory/tests/Base64/Base64DecoderUnitTests.cs	Added test case DecodingWithEmbeddedWhiteSpaceIntoSmallDestination_TrailingWhiteSpacesAreConsumed to verify trailing whitespace is properly consumed even with small destination buffers
src/libraries/System.Runtime/tests/System.Runtime.Extensions.Tests/System/Convert.cs	Added assertions to existing parameterized tests to verify Base64.DecodeFromUtf8 behaves consistently with Convert methods across various input patterns