[release/9.0-staging] Detect if RSA-384 is supported on the platform

[vcsjones]

Backport of #120366 to release/9.0-staging

/cc @bartonjs

Customer Impact

• Customer reported
• Found internally

This is test-only reaction to a change in Windows. Tests started failing in Windows because a Windows Update caused RSA with a 384-bit modulus to fail.

Regression

• Yes
• No

Testing

This is test only change. Verified failing tests started passing again.

Risk

None. Test-only change.

[Copilot]

Pull Request Overview

Backport test-only change to dynamically detect RSA-384 key support instead of using OS string heuristics, stabilizing previously failing cryptography tests on updated Windows versions.

• Centralizes RSA-384 capability detection in PlatformSupport.IsRSA384Supported.
• Replaces per-provider ad hoc logic with a shared Lazy-based capability probe.
• Adds embedded 384-bit RSA key parameters for runtime import testing on Windows.

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
System.Security.Cryptography/tests/DefaultRSAProvider.cs	Replaces cached OSDescription heuristic with PlatformSupport.IsRSA384Supported.
System.Security.Cryptography.Csp/tests/RSACryptoServiceProviderProvider.cs	Uses new shared RSA-384 support detection instead of unconditional true.
System.Security.Cryptography.Cng/tests/RSACngProvider.cs	Removes duplicated heuristic logic in favor of centralized detection.
Common/tests/System/Security/Cryptography/PlatformSupport.cs	Adds RSA-384 parameter set and Lazy-based runtime capability detection exposed via IsRSA384Supported.

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[bartonjs]

Test-only change, tell-mode.