Assert DirectE2EE mode on Connect and Bind

[andrewpmartinez]

Assert the DirectE2EE mode on Connect and Bind and echo the agreed mode on the responses, because the legacy-versus-DirectE2EE decision has to be made before any TLS exists.

• Send DirectE2EEMode on Connect and Bind, independent of whether a libsodium public key is present
• Read the agreed mode from StateConnected and BindSuccess
• Fall back to legacy under permissive when the peer asserts no mode
• Reject and surface E2eeNoCommonMode when no mode is shared
• Map a legacy peer rejection of a missing public key onto E2eeNoCommonMode