Add error for when an etag is specified in dashboard configuration (#3723)

## Changes
Users should never set etags in their configuration. It's meant to be a
pure internal state.

## Why
This validation can potentially help simplify downstream code since we
can read etags directly from a merged view of (config + state) and be
confident that the etag values come from state.

Note: This is not a breaking change since existing users would see
errors when they try to update their dashboard if they specified etags
in their configuration today:
```
databricks_dashboard.dashboard: Modifying... [id=01f0a29746cd1d1991ae7cc9e6331576]
╷
│ Error: cannot update dashboard: Entity etag provided does not match currently stored etag. Please reload the entity and try the action again.
│ 
│   with databricks_dashboard.dashboard,
│   on main.tf line 13, in resource "databricks_dashboard" "dashboard":
│   13: resource "databricks_dashboard" "dashboard" {
│ 
╵
```

## Tests
New acceptance test.

Author: shreyas-goenka

NEXT_CHANGELOG.md

@@ -11,6 +11,7 @@
 ### Bundles
 * Fix processing short pip flags in environment dependencies ([#3708](https://github.com/databricks/cli/pull/3708))
 * Add support for referencing local files in -e pip flag for environment dependencies ([#3708](https://github.com/databricks/cli/pull/3708))
+* Add error for when an etag is specified in dashboard configuration. Setting etags was never supported / valid in bundles but now users will see this error during validation rather than deployment. ([#3723](https://github.com/databricks/cli/pull/3723))
 * Fix PIP flag processing in pipeline environment dependencies ([#3734](https://github.com/databricks/cli/pull/3734))
 
 ### API Changes

acceptance/bundle/validate/no_dashboard_etag/databricks.yml

@@ -0,0 +1,6 @@
+resources:
+  dashboards:
+    foobar:
+      display_name: foobar
+      etag: "1234567890"
+      serialized_dashboard: "{}"

acceptance/bundle/validate/no_dashboard_etag/out.test.toml

@@ -0,0 +1,15 @@
+
+>>> [CLI] bundle validate
+Error: dashboard "foobar" has an etag set. Etags must not be set in bundle configuration
+  at resources.dashboards.foobar
+  in databricks.yml:6:7
+
+Name: test-bundle
+Target: default
+Workspace:
+  User: [USERNAME]
+  Path: /Workspace/Users/[USERNAME]/.bundle/test-bundle/default
+
+Found 1 error
+
+Exit code: 1

acceptance/bundle/validate/no_dashboard_etag/output.txt

@@ -0,0 +1 @@
+trace $CLI bundle validate

acceptance/bundle/validate/no_dashboard_etag/script

@@ -0,0 +1,37 @@
+package validate
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/databricks/cli/bundle"
+	"github.com/databricks/cli/libs/diag"
+	"github.com/databricks/cli/libs/dyn"
+)
+
+func ValidateDashboardEtags() bundle.ReadOnlyMutator {
+	return &validateDashboardEtags{}
+}
+
+type validateDashboardEtags struct{ bundle.RO }
+
+func (v *validateDashboardEtags) Name() string {
+	return "validate:validate_dashboard_etags"
+}
+
+func (v *validateDashboardEtags) Apply(ctx context.Context, b *bundle.Bundle) diag.Diagnostics {
+	// No dashboards should have etags set. They are purely internal state.
+	for k, dashboard := range b.Config.Resources.Dashboards {
+		if dashboard.Etag != "" {
+			return diag.Diagnostics{
+				{
+					Severity:  diag.Error,
+					Summary:   fmt.Sprintf("dashboard %q has an etag set. Etags must not be set in bundle configuration", dashboard.DisplayName),
+					Paths:     []dyn.Path{dyn.MustPathFromString("resources.dashboards." + k)},
+					Locations: b.Config.GetLocations("resources.dashboards." + k),
+				},
+			}
+		}
+	}
+	return nil
+}

bundle/config/validate/validate_dashboard_etags.go

@@ -142,6 +142,9 @@ func Initialize(ctx context.Context, b *bundle.Bundle) {
 		// Validate that all fields with enum values specified are set to a valid value.
 		validate.Enum(),
 
+		// Validate that no dashboard etags are set. They are purely internal state and should not be set by the user.
+		validate.ValidateDashboardEtags(),
+
 		// Reads (typed): b.Config.Permissions (checks if current user or their groups have CAN_MANAGE permissions)
 		// Reads (typed): b.Config.Workspace.CurrentUser (gets current user information)
 		// Provides diagnostic recommendations if the current deployment identity isn't explicitly granted CAN_MANAGE permissions