Implement KeySizeInBytes on AesGcm and AesCcm.

[Copilot]

Implements #72191

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[Copilot]

Pull request overview

This PR adds a new public instance property, KeySizeInBytes, to AesGcm and AesCcm, allowing callers to query the key length used to construct an instance.

Changes:

• Introduces KeySizeInBytes as a public property on AesGcm and AesCcm.
• Initializes KeySizeInBytes in all relevant constructors (including obsolete AesGcm ctors).
• Adds unit tests validating KeySizeInBytes for 128/192/256-bit keys and updates the ref assembly surface.

Show a summary per file

File	Description
src/libraries/System.Security.Cryptography/tests/AesCcmTests.cs	Adds coverage asserting AesCcm.KeySizeInBytes matches the provided key length.
src/libraries/System.Security.Cryptography/src/System/Security/Cryptography/AesGcm.cs	Ensures the obsolete AesGcm(ReadOnlySpan<byte>) constructor populates KeySizeInBytes.
src/libraries/System.Security.Cryptography/src/System/Security/Cryptography/AesCcm.cs	Adds KeySizeInBytes API and sets it in both constructors.
src/libraries/System.Security.Cryptography/ref/System.Security.Cryptography.cs	Exposes KeySizeInBytes in the public ref contract for both AesGcm and AesCcm.
src/libraries/Common/tests/System/Security/Cryptography/AesGcmTests.cs	Adds coverage asserting AesGcm.KeySizeInBytes across both obsolete and tag-size ctors (as applicable).
src/libraries/Common/src/System/Security/Cryptography/AesGcm.cs	Adds KeySizeInBytes API and sets it in the tag-size constructor.

Copilot's findings

• Files reviewed: 6/6 changed files
• Comments generated: 0