Add audit logging for MCP server OAuth token storage

What does this MR do and why?

Add audit logging for MCP server OAuth token storage

Emit a mcp_server_oauth_token_created audit event from Ai::Catalog::McpServers::CallbackService after a user successfully connects to an MCP server and their OAuth token is stored, providing visibility into which users have connected to which MCP servers.

References

https://gitlab.com/gitlab-org/gitlab/-/work_items/593029

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