agent: allow setting auto-auth and vault.namespace separately (#26427)

The new vault.namespace config option seems to always be overridden by
the auto-auth namespace, so add an extra check to allow them to be set
separately.

---------

Co-authored-by: Violet Hynes <violet.hynes@hashicorp.com>

changelog/26427.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+agent: `vault.namespace` no longer gets incorrectly overridden by `auto_auth.namespace`, if set
+```

command/agent.go

@@ -328,6 +328,9 @@ func (c *AgentCommand) Run(args []string) int {
 			client.SetNamespace(config.AutoAuth.Method.Namespace)
 		}
 		templateNamespace = client.Namespace()
+		if !namespaceSetByEnvironmentVariable && config.Vault != nil && config.Vault.Namespace != "" {
+			templateNamespace = config.Vault.Namespace
+		}
 
 		sinkClient, err := client.CloneWithHeaders()
 		if err != nil {