Verify the VBS enclave key binding on the async attestation path

[virzak]

CreateEnclaveSessionCoreAsync derives the session secret without calling VerifyEnclavePublicKeyBinding. The check was added to the sync path in #4532, and the async path added in #4541 doesn't call it. Nothing calls the async path yet, so no release is affected. @cheenamalhotra for context, as this is your async enclave work.

The check now runs inside GetSharedSecret, so neither path can derive a secret from a key the signed report doesn't commit to.

Testing

• New GetSharedSecret_UnboundKey_Throws: rejects an unbound key before using the Diffie-Hellman inputs. Fails without the fix.
• Enclave unit tests pass on net8.0 and net462.
• Not added: an end-to-end async test, which needs a signed HGS health report and enclave report fixture.

[azure-pipelines]

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