Fixup a few dataprotection and antiforgery edge cases

[BrennanConroy]

With the recent dataprotection issue, I had AI look at the dataprotection code a bit more closely and found a few edge cases we can handle better.

1. DefaultAntiforgeryTokenGenerator should be checking the ClaimUidBytes regardless of whether the original token has any, this lets us validate that if a different user connects with a previous token (or the same user but they have new claims and so token doesn't match)
2. DefaultAntiforgeryTokenGenerator was using SequenceEqual when it should be using a constant-time comparison
3. Read7BitEncodedInt didn't handle int32 overflow (it just returned 0). FormatterBinaryReader.Read7BitEncodedInt throws FormatException in this case, so updated to have the same behavior
4. The span-based protect/unprotect methods assumed a span-based encryptor from the key ring. While that should be the case 99.99% of the time, it's technically not required to be true, so added a fallback path
5. There were some stackalloc byte[255] which bothered me a bit, 256 is the normal value we use 😆

[Copilot]

Pull request overview

Handles several edge cases across antiforgery and data protection, aligning behavior with framework expectations and tightening validation logic.

Changes:

• Antiforgery: always attempt claim UID extraction during validation and use constant-time claim UID comparison.
• Shared encoding: make Read7BitEncodedInt throw FormatException on int32 overflow (including invalid 5th byte).
• DataProtection: add fallback paths when span-based protect/unprotect is used with a non-span encryptor; normalize small stackalloc buffers to 256 bytes.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
src/Shared/test/Shared.Tests/Encoding/Int7BitEncodingUtilsTests.cs	Adds regression test for 5th-byte overflow in 7-bit int decoding.
src/Shared/Encoding/Int7BitEncodingUtils.cs	Adds explicit overflow detection for invalid 5th byte in 7-bit int decoding.
src/DataProtection/benchmarks/Microsoft.AspNetCore.DataProtection.MicroBenchmarks/Benchmarks/SpanDataProtectorComparison.cs	Updates benchmark stackalloc size from 255 to 256.
src/DataProtection/DataProtection/test/Microsoft.AspNetCore.DataProtection.Tests/Managed/ManagedAuthenticatedEncryptorTests.cs	Updates test comment to reflect 256-byte initial stackalloc size.
src/DataProtection/DataProtection/test/Microsoft.AspNetCore.DataProtection.Tests/KeyManagement/KeyRingBasedDataProtectorTests.cs	Adds regression tests for span protector fallback when encryptor isn’t span-capable.
src/DataProtection/DataProtection/src/Managed/ManagedAuthenticatedEncryptor.cs	Uses stackalloc[256] for output sizes up to and including 256.
src/DataProtection/DataProtection/src/Managed/AesGcmAuthenticatedEncryptor.cs	Uses stackalloc[256] for output sizes up to and including 256.
src/DataProtection/DataProtection/src/KeyManagement/KeyRingBasedSpanDataProtector.cs	Adds fallback to byte[] path when span encryptor is unexpectedly unavailable.
src/DataProtection/DataProtection/src/Cng/CngGcmAuthenticatedEncryptor.cs	Uses stackalloc[256] for output sizes up to and including 256.
src/DataProtection/DataProtection/src/Cng/CbcAuthenticatedEncryptor.cs	Uses stackalloc[256] for output sizes up to and including 256.
src/Antiforgery/test/DefaultAntiforgeryTokenGeneratorTest.cs	Adds regression test for username token validation when user now has extractable claims; updates one existing test setup.
src/Antiforgery/src/Internal/DefaultAntiforgeryTokenGenerator.cs	Always extracts claim UID bytes for authenticated users and switches to constant-time UID comparison.

[Copilot]

AreIdenticalClaimUids can reach FixedTimeEquals(token.ClaimUid.GetData(), claimUidBytes) even when claimUidBytesExtracted is false. Since claimUidBytes is stackalloc'd and not initialized when extraction fails, this risks nondeterministic behavior (and, in theory, a false positive). Consider returning false immediately when token.ClaimUid is non-null but claimUidBytesExtracted is false, before the length check / compare.