agent: Use inventory for AgentPreview (#30740)

This PR updates the `AgentPreview` to use `inventory` instead of
`linkme`.

Release Notes:

- N/A

crates/ui_macros/Cargo.toml

@@ -14,7 +14,6 @@ proc-macro = true
 
 [dependencies]
 convert_case.workspace = true
-linkme.workspace = true
 proc-macro2.workspace = true
 quote.workspace = true
 syn.workspace = true