fix: render base variables into templates instead of tfvars (DEVEX-287) (#26436)

Part of the Template Builder wizard PR stack.

## Problem

The kubernetes base template used Terraform `variable` blocks and
`var.*` references for `use_kubeconfig` and `namespace`, but the
composed tar bundle never included a `.tfvars` file. This caused
`terraform plan` to fail with "required template variables need values:
namespace".

## Fix

Base templates now use Go template variables (`{{ .Variables.* }}`) just
like module templates do. Values are validated, HCL-quoted, and rendered
directly into the output HCL.

Also adds explicit "variable is required" validation to both
`mergeBaseVariables` and `mergeModuleVariables`, replacing the previous
reliance on `missingkey=error` at render time for clearer error
messages.

---
> [!NOTE]
> Generated by Coder Agents on behalf of @jeremyruppel

coderd/templatebuilder/bases.go

@@ -174,9 +174,24 @@ func DefaultBaseRenderContext(exampleID string) BaseRenderContext {
 	if err != nil || bases[exampleID] == nil {
 		return BaseRenderContext{}
 	}
-	dc := bases[exampleID].Manifest.DefaultContext
+	base := bases[exampleID]
+	dc := base.Manifest.DefaultContext
+
+	// Populate Variables from manifest defaults so that Go template
+	// rendering succeeds even without caller-supplied values.
+	vars := make(map[string]string, len(base.Manifest.Variables))
+	for _, v := range base.Manifest.Variables {
+		if v.Computed || v.Sensitive {
+			continue
+		}
+		if len(v.Default) > 0 && isSimpleJSONValue(v.Default) {
+			vars[v.Name] = string(v.Default)
+		}
+	}
+
 	return BaseRenderContext{
 		ContainerImage: dc.ContainerImage,
+		Variables:      vars,
 	}
 }