feat: report synthetic metadata from fake agents (#25166)

Fake agents now fetch their manifest, spawn a single per-agent metadata
goroutine, and emit batched BatchUpdateMetadata calls with 3072-byte
base64 payloads so scaletest runs mirror the load shape of real agents.
This matches what the current scaletest workspace template does for
metadata. In the future we can extend the harness here to take in a
config option for the metadata payload size.

---------

Signed-off-by: Callum Styan <callumstyan@gmail.com>
Co-authored-by: Mux <mux@coder.com>

enterprise/cli/exp_scaletest_agentfake.go

@@ -68,7 +68,7 @@ func (r *RootCmd) scaletestAgentFake() *serpent.Command {
 			}
 
 			logger := inv.Logger
-			mgr := agentfake.NewManager(client, logger, agentfake.ManagerOptions{
+			mgr := agentfake.NewManager(client.URL, client, logger, agentfake.ManagerOptions{
 				Template: template,
 				Owner:    owner,
 			})