Update self-hosted models generations and suggestions to use v3 code suggestions endpoint

This issue is to bring the self-hosted version of Code Suggestions to parity with the .com version of Code Suggestions, to include incorporation of additional context sources.

Definition of Done

• Migrate AiGatewaySelfHostedMessages to match or replace it with AiGatewayMessages.
• Ensure that the self-hosted Code Suggestion feature using all supported models works as expected for both code completion and code generation.
• Ensure feature parity, in terms of context accessible, for self-hosted Code Suggestions