Draft: Resolve "[UX] Improve visibility and source project UI copy"

What does this MR do and why?

Addresses the UI copy and presentation issues described in #578610. Additional background here: https://gitlab.com/gitlab-org/gitlab/-/issues/578131#note_2843563994

Changes:

• Reword labels and help text for both inputs in the Visibility & Access section.
• Reword Private / Public visibility options to Project members / All users and remove their icons.
• Update popover text to reflect terminology changes.
• Move the Managed by project selection (previously Source project) above the Visibility radio buttons.
• Small style fix to remove extra margin between radio buttons that resulted in excess spacing.

TBD:

• We should also update the conditional alerts that are displayed when creating or editing an agent. First need to clarify the meaning of "used" in This agent can be made private if it's not used and A public agent can be made private only if it's not used.
• Also will impact private/public labels on the table list in https://gitlab.com/gitlab-org/gitlab/-/issues/577303

References

Screenshots or screen recordings

Screen_Recording_2025-10-28_at_8.46.15_PM

Before	After
	TBD

How to set up and validate locally

MR acceptance checklist

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Related to #578610