Feature-detect KMAC key size limits

[vcsjones]

KMAC on OpenSSL's default provider has key size limits, the key must be in the range of [4, 512]. Windows does not have this limit, and SymCrypt-OpenSSL does not have this limit, either.

Detecting the presence of which provider is the default is tricky; we can look for a provider's presence on the system, but which is active is not always clear, and sometimes one provider defers to another.

This changes our KMAC tests to do a check it see if can import a zero-length key to determine the key size limits, rather than trying to detect provider presence.

Contributes to #129939

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[Copilot]

Pull request overview

This PR updates the KMAC test infrastructure to infer OpenSSL key-size restrictions by probing whether a zero-length key can be imported, instead of attempting to identify the active/default OpenSSL provider via platform detection heuristics.

Changes:

• Replace provider-based detection of KMAC key size limits with a runtime probe (Create with an empty key).
• Compute PlatformKeySizeRequirements from the probe result to drive test expectations and the default MinimalKey.

Show a summary per file

File	Description
src/libraries/System.Security.Cryptography/tests/KmacTestDriver.cs	Adds a runtime probe to detect OpenSSL KMAC key-size limits and uses it to set PlatformKeySizeRequirements.

Copilot's findings

• Files reviewed: 1/1 changed files
• Comments generated: 1