Backport SHR p-claim percent-encoding hex case handling to dev8x

[debchoudhury-id4s]

Summary

Backports #3561 to the dev8x branch.

• treats hexadecimal letters inside valid percent-encoded triplets as case-insensitive during SHR p claim validation
• preserves the 8.x default for literal path casing through UseCaseSensitivePClaimComparison
• keeps encoded characters distinct from literal delimiters and preserves p claim creation output
• includes the corresponding validation and creation tests

Replaces the fork-based PR #3578.

Testing

• Targeted ValidatePClaim and CreatePClaim tests pass on .NET 8 and 10 (66 tests per target)
• .NET 6 and 9 require CI because those runtimes are not installed locally
• .NET Framework targets require an environment configured for delay-signed assembly verification

[Copilot]

Pull request overview

Backports the Signed HTTP Request (SHR) p claim validation fix to the dev8x line so that percent-encoded triplets compare equivalently regardless of hex-letter casing (e.g., %2F == %2f) while preserving the existing UseCaseSensitivePClaimComparison behavior for literal path casing and keeping creation output unchanged.

Changes:

• Updates SignedHttpRequestHandler.ValidatePClaim to perform an additional percent-triplet-aware comparison when ordinal comparison fails.
• Expands SHR p claim validation and creation test coverage for percent-encoding hex-case scenarios and delimiter-vs-encoded character distinctions.
• Adds a changelog entry describing the behavior change.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/Microsoft.IdentityModel.Protocols.SignedHttpRequest/SignedHttpRequestHandler.cs	Implements percent-triplet-aware equality for p claim validation (case-insensitive hex letters only within valid %XX triplets).
test/Microsoft.IdentityModel.Protocols.SignedHttpRequest.Tests/SignedHttpRequestValidationTests.cs	Adds comparison-matrix coverage and exception-message casing preservation tests for ValidatePClaim.
test/Microsoft.IdentityModel.Protocols.SignedHttpRequest.Tests/SignedHttpRequestCreationTests.cs	Adds tests ensuring p claim creation preserves percent-encoding hex casing and double-encoding.
CHANGELOG.md	Documents the bug fix in the 8.x changelog.