feat: add GET /api/v2/templatebuilder/modules endpoint (#26117)

Implement `GET /api/v2/templatebuilder/modules`, which returns the
filtered list of modules available for a given base template. Reads from
the bundled catalog via `LoadModules()` and applies OS-compatibility
filtering based on the `base` query param.

Computed variables (e.g. `agent_id`) are excluded from the API response
at the `ToSDK()` conversion boundary since they are wired automatically
by the builder. The `Computed` field is removed from the SDK type. Adds
`CompatibleWithOS()` to `ModuleManifest` for OS filtering.

Returns 400 for unknown base IDs and 404 when the template builder is
disabled.

Depends on #26116

> [!NOTE]
> This PR was authored by Coder Agents on behalf of @jeremyruppel.

coderd/templatebuilder/catalog.go

@@ -137,11 +137,31 @@ func parseModulesFromFS(fsys fs.FS) ([]ModuleManifest, error) {
 	return modules, nil
 }
 
+// CompatibleWithOS reports whether the module is compatible with the given OS.
+// Modules with an empty CompatibleOS list are compatible with all platforms.
+func (m ModuleManifest) CompatibleWithOS(os string) bool {
+	if len(m.CompatibleOS) == 0 {
+		return true
+	}
+	for _, supported := range m.CompatibleOS {
+		if supported == os {
+			return true
+		}
+	}
+	return false
+}
+
 // ToSDK converts a ModuleManifest to the API response type.
 // PinnedVersion is mapped to Version; tags are not part of the API surface.
+// Computed variables are excluded from the output.
 func (m ModuleManifest) ToSDK() codersdk.TemplateBuilderModule {
 	variables := make([]codersdk.TemplateBuilderModuleVariable, 0, len(m.Variables))
 	for _, v := range m.Variables {
+		// Computed variables (e.g. agent_id) are wired by the builder
+		// automatically and must not be surfaced to the user.
+		if v.Computed {
+			continue
+		}
 		variables = append(variables, codersdk.TemplateBuilderModuleVariable{
 			Name:        v.Name,
 			Type:        validVariableTypes[v.Type],
@@ -149,7 +169,6 @@ func (m ModuleManifest) ToSDK() codersdk.TemplateBuilderModule {
 			Default:     v.Default,
 			Required:    v.Required,
 			Sensitive:   v.Sensitive,
-			Computed:    v.Computed,
 		})
 	}
 

coderd/templatebuilder/catalog_test.go

@@ -93,18 +93,10 @@ func TestToSDK(t *testing.T) {
 	t.Run("AllVariableFields", func(t *testing.T) {
 		t.Parallel()
 
-		require.Len(t, sdk.Variables, 3)
+		// Computed variables (agent_id) are excluded from SDK output.
+		require.Len(t, sdk.Variables, 2)
 
-		agent := sdk.Variables[0]
-		require.Equal(t, "agent_id", agent.Name)
-		require.Equal(t, codersdk.TemplateBuilderVariableTypeString, agent.Type)
-		require.Equal(t, "The Coder agent ID.", agent.Description)
-		require.Nil(t, agent.Default)
-		require.True(t, agent.Required)
-		require.False(t, agent.Sensitive)
-		require.True(t, agent.Computed)
-
-		port := sdk.Variables[1]
+		port := sdk.Variables[0]
 		require.Equal(t, "port", port.Name)
 		require.Equal(t, codersdk.TemplateBuilderVariableTypeNumber, port.Type)
 		require.Equal(t, "Port to listen on.", port.Description)
@@ -112,16 +104,14 @@ func TestToSDK(t *testing.T) {
 		require.JSONEq(t, "8080", string(port.Default))
 		require.False(t, port.Required)
 		require.False(t, port.Sensitive)
-		require.False(t, port.Computed)
 
-		secret := sdk.Variables[2]
+		secret := sdk.Variables[1]
 		require.Equal(t, "secret_key", secret.Name)
 		require.Equal(t, codersdk.TemplateBuilderVariableTypeString, secret.Type)
 		require.Equal(t, "A sensitive value.", secret.Description)
 		require.Nil(t, secret.Default)
 		require.True(t, secret.Required)
 		require.True(t, secret.Sensitive)
-		require.False(t, secret.Computed)
 	})
 
 	t.Run("NilSlicesNormalizedToEmpty", func(t *testing.T) {
@@ -141,3 +131,32 @@ func TestToSDK(t *testing.T) {
 		require.Empty(t, s.Variables)
 	})
 }
+
+func TestCompatibleWithOS(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name         string
+		compatibleOS []string
+		os           string
+		want         bool
+	}{
+		{"EmptyListMatchesAll", nil, "linux", true},
+		{"ExactMatch", []string{"linux"}, "linux", true},
+		{"MultipleMatch", []string{"linux", "windows"}, "windows", true},
+		{"NoMatch", []string{"linux"}, "windows", false},
+		{"CaseSensitive", []string{"linux"}, "Linux", false},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+			m := templatebuilder.ModuleManifest{
+				ID:            "test",
+				PinnedVersion: "1.0.0",
+				CompatibleOS:  tc.compatibleOS,
+			}
+			require.Equal(t, tc.want, m.CompatibleWithOS(tc.os))
+		})
+	}
+}