Add System.Security.Cryptography Copilot instructions

[PranavSenthilnathan]

Adds path-specific GitHub Copilot guidance for System.Security.Cryptography, covering established code style, correctness, sensitive-data handling, constant-time comparisons, and test skipping conventions.

Documentation-only change; no build or tests were run.

Note

This pull request description was generated by GitHub Copilot.

[azure-pipelines]

Azure Pipelines:
Successfully started running 1 pipeline(s).
15 pipeline(s) were filtered out due to trigger conditions.
There may be pipelines that require an authorized user to comment /azp run to run.

[dotnet-policy-service]

Tagging subscribers to this area: @bartonjs, @vcsjones, @dotnet/area-system-security
See info in area-owners.md if you want to be subscribed.

[Copilot]

Pull request overview

Adds a new folder-scoped GitHub Copilot instruction file for src/libraries/System.Security.Cryptography/** to guide generated contributions toward established crypto coding and testing patterns.

Changes:

• Introduces System.Security.Cryptography-specific guidance covering code style, correctness checks for Try* APIs, and sensitive-data handling.
• Adds security recommendations around constant-time comparisons and buffer clearing/returning.
• Adds test-skipping guidance intended to reduce noisy skips (but needs adjustment to match existing patterns/semantics).

[PranavSenthilnathan]

Hopefully these comments go away with the new instructions.

[Copilot]

Pull request overview

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

Comments suppressed due to low confidence (1)

.github/instructions/system-security-cryptography.instructions.md:24

• Returning early from an xUnit test reports the test as passed, not skipped, so this guidance can hide missing coverage. Also, System.Security.Cryptography tests already use SkipTestException widely (e.g., CryptoConfigTests.cs:158 and SkipTestException.ThrowUnless(...) in ECDsaCngImportExportTests.cs:56), so advising against it unconditionally doesn’t match established practice. Suggest: prefer [ConditionalFact]/[ConditionalTheory] when possible, otherwise use SkipTestException so the test is reported as skipped.

- Avoid throwing `SkipTestException`, which creates noisy test output. Prefer `[ConditionalFact]` or `[ConditionalTheory]` with a condition; when that is not possible, return early from the test.