Show agent type in the AI audit event report

What does this MR do and why?

The AI Audit Event Report hardcoded every session's client type to "GitLab Duo". Now that the GraphQL query supports agent type, we can use that to render the proper "client type"

• Humanises unknown agent types (for example some-agent becomes "Some agent"), so the report never attributes a session to the wrong tool, including agent types added to the backend later.
• Renders agent types without an icon as a circular GlAvatar initial (the same treatment used in list_selector/user_item.vue and project_avatar.vue), via a new ClientTypeIcon component shared by the table's AI Item cell and the session details drawer's "Client type" row.
• In the future, we'll be adding the official logos for these third party agent types so that we can render those icons instead of the circle avatar.

References

• Closes https://gitlab.com/gitlab-org/gitlab/-/issues/628370
• Backend field: !252345 (merged) (for https://gitlab.com/gitlab-org/gitlab/-/issues/623440)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. On a GDK with the agent_artifacts_page feature flag enabled and AI audit event storage turned on, seed one Duo Agent Platform session (agent_type: null) and one external session (for example agent_type: 'claude-code').
2. Visit the group's audit events report at http://127.0.0.1:3000/groups/<group>/-/audit_events and confirm the AI Item column and the session details drawer show "GitLab Duo" for the Duo Agent Platform session and "Claude Code" with an initial avatar for the external session.

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