Fix ECDSA key registration in Commander SSH agent

[rporter92]

Problem

Commander's built-in SSH agent (ssh-agent start then ssh <record>) fails to add any ecdsa-sha2-* key:

SSH request process error: type object
'cryptography.hazmat.primitives.asymmetric.ec.EllipticCurvePublicNumbers'
has no attribute 'from_encoded_point'
Add ssh-key. Failed to add ssh key "<record>" to ssh-agent

Two issues in the ECDSA branch of the ADD_IDENTITY handler in keepercommander/commands/ssh_agent.py:

1. EllipticCurvePublicNumbers.from_encoded_point() was removed in cryptography 39.0.0; requirements.txt requires >=46.0.6, so this raises on every supported install.
2. The stored public key blob omits the curve-name string required by RFC 5656 §3.1. Previously masked by (1); once (1) is fixed alone, ssh-add -l reports elliptic curve does not match and authentication fails.

Fix

• Use EllipticCurvePublicKey.from_encoded_point(curve, point).public_numbers().
• Include curve_name in key.key_blob.

No dependency change; the replacement API exists in every cryptography version Commander has ever required.

Testing

• Existing unit-tests/ suite passes.
• Manually created ECDSA Key after change applied and confirmed ECDSA key is now accepted in Commander SSH Agent. Errors no longer occurring.