feat: hide deprecated AppKit plugins from apps init picker (#6834)

## Summary

AppKit now emits `deprecated: true` on plugin entries in
`appkit.plugins.json` ([AppKit PR
#596](databricks/appkit#596)). This makes
`databricks apps init` skip deprecated plugins in the "Select features"
picker.

Two changes in `libs/apps/manifest/manifest.go`:
- Add `Deprecated bool` to the `Plugin` struct.
- Exclude deprecated plugins in `GetSelectablePlugins()`. A deprecated
plugin is then neither selectable nor mandatory, so it drops out of the
picker entirely.

## Testing

- Added `TestGetSelectablePluginsExcludesDeprecated` in
`libs/apps/manifest`.
- `go build ./...`
- `go test ./libs/apps/... ./cmd/apps/...`

This pull request and its description were written by Isaac.

---------

Co-authored-by: MarioCadenas <MarioCadenas@users.noreply.github.com>
Co-authored-by: Isaac <no-reply@databricks.com>

Author: MarioCadenas

libs/apps/manifest/manifest.go

@@ -85,6 +85,10 @@ type Plugin struct {
 	// Stored as a plain string so unknown future values round-trip unchanged.
 	// See https://github.com/databricks/appkit/pull/264.
 	Stability string `json:"stability,omitempty"`
+
+	// Deprecated plugins are hidden from the init picker.
+	// See https://github.com/databricks/appkit/pull/596.
+	Deprecated bool `json:"deprecated,omitempty"`
 }
 
 // StabilityLabel returns a user-facing tier label for non-GA plugins.
@@ -153,7 +157,7 @@ func (m *Manifest) GetPlugins() []Plugin {
 func (m *Manifest) GetSelectablePlugins() []Plugin {
 	var selectable []Plugin
 	for _, p := range m.GetPlugins() {
-		if !p.RequiredByTemplate {
+		if !p.RequiredByTemplate && !p.Deprecated {
 			selectable = append(selectable, p)
 		}
 	}

libs/apps/manifest/manifest_test.go

@@ -169,6 +169,24 @@ func TestGetSelectablePlugins(t *testing.T) {
 	assert.Equal(t, "optional-plugin", selectable[1].Name)
 }
 
+func TestGetSelectablePluginsExcludesDeprecated(t *testing.T) {
+	m := &manifest.Manifest{
+		Plugins: map[string]manifest.Plugin{
+			"analytics": {
+				Name: "analytics",
+			},
+			"legacy": {
+				Name:       "legacy",
+				Deprecated: true,
+			},
+		},
+	}
+
+	selectable := m.GetSelectablePlugins()
+	require.Len(t, selectable, 1)
+	assert.Equal(t, "analytics", selectable[0].Name)
+}
+
 func TestGetMandatoryPlugins(t *testing.T) {
 	m := &manifest.Manifest{
 		Plugins: map[string]manifest.Plugin{