[patch] Reject whitespace in [IsBase64] regardless of total length

[matt-edmondson]

Fixes #316

What was wrong

[IsBase64] checked two things: that value.Length % 4 == 0, and that Convert.FromBase64String(value) didn't throw. FromBase64String silently skips spaces, tabs, CR and LF, but the length check counts them. So a value containing whitespace passed whenever the whitespace made the total length a multiple of 4, and it was stored verbatim. The existing IsBase64Attribute_Base64WithSpaces_ShouldThrow only passed because its sample happened to be 19 characters long.

Change (Semantics.Strings/Validation/Attributes/Text/IsBase64Attribute.cs)

• After the length check, the value must use only the Base64 alphabet (A-Z, a-z, 0-9, +, /), followed by at most two = at the end. Anything else fails with the same "must be a valid Base64 string" message.
• Convert.FromBase64String still runs after that to confirm the padding.

Tests (TextValidationAttributesTests)

• The whitespace test is now data-driven, and its samples no longer depend on length parity. The original 19-character sample is kept, plus the three 20-character values from the issue that used to be accepted (trailing spaces, embedded spaces plus a trailing space, and CRLF), and trailing tabs.
• IsBase64Attribute_MisplacedPadding_ShouldThrow: = mid-value, a non-= after =, and three =.
• IsBase64Attribute_PlusAndSlash_ShouldPass: confirms + and / are still accepted.
• The existing valid vectors (including the empty string) still pass, unchanged.

With the fix reverted, 4 of the whitespace cases fail. With the fix, the full Semantics.Test suite passes: 1489 passed, 0 failed, 8 skipped. Semantics.Strings builds for every target framework.

🤖 Generated with Claude Code

https://claude.ai/code/session_019RYN9jT6szUiySkywFKCA1

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

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