Add refinements to the AI onboarding flow (#33738)

This includes making sure that both the agent panel and Zed's edit
prediction have a consistent narrative when it comes to onboarding users
into the AI features, considering the possible different plans and
conditions (such as being signed in/out, account age, etc.)

Release Notes:

- N/A

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Co-authored-by: Bennet Bo Fenner <53836821+bennetbo@users.noreply.github.com>
Co-authored-by: Bennet Bo Fenner <bennetbo@gmx.de>

crates/agent_ui/Cargo.toml

@@ -21,6 +21,7 @@ agent.workspace = true
 agentic-coding-protocol.workspace = true
 agent_settings.workspace = true
 agent_servers.workspace = true
+ai_onboarding.workspace = true
 anyhow.workspace = true
 assistant_context.workspace = true
 assistant_slash_command.workspace = true