Do not check required fields inside a pure reference (#7226)

## Changes
The required-field warning check (`bundle/config/validate/required.go`)
skips a value that is a pure `${...}` reference.

## Why
`schedule: ${resources.jobs.base.schedule}` produced `required field
"quartz_cron_expression" is not set` (and `timezone_id`) for the
referring job, although the referenced schedule sets both. The same
happens on main for e.g. `queue: ${...}`. The referenced value is
checked where it is defined.

Stacked on #7225 (the test only
gets past validate with that fix).

Port note: the fix lives in `warnForMissingFields` and is dyn-specific
(it walks the dyn tree); a typed walker needs the same skip for fields
held as references.

## Tests
- Unit test in `bundle/config/validate/required_test.go`.
- `acceptance/bundle/validate/references/job_schedule_whole_reference`:
warnings gone, Badness removed.

This pull request and its description were written by Isaac.

---------

Co-authored-by: Isaac <no-reply@databricks.com>

Author: denik

.nextchanges/bundles/required-skip-reference.md

@@ -0,0 +1 @@
+* Removed spurious "required field is not set" warnings for a field whose value is a reference to a whole block, such as `schedule: ${resources.jobs.base.schedule}`. ([#7226](https://github.com/databricks/cli/pull/7226))

acceptance/bundle/validate/references/job_schedule_whole_reference/output.txt

@@ -1,30 +1,14 @@
 
 >>> [CLI] bundle validate
-Warning: required field "quartz_cron_expression" is not set
-  at resources.jobs.viaref.schedule
-  in databricks.yml:9:17
-
-Warning: required field "timezone_id" is not set
-  at resources.jobs.viaref.schedule
-  in databricks.yml:9:17
-
 Name: test-bundle
 Target: default
 Workspace:
   User: [USERNAME]
   Path: /Workspace/Users/[USERNAME]/.bundle/test-bundle/default
 
-Found 2 warnings
+Validation OK!
 
 >>> [CLI] bundle plan -o json
-Warning: required field "quartz_cron_expression" is not set
-  at resources.jobs.viaref.schedule
-  in databricks.yml:9:17
-
-Warning: required field "timezone_id" is not set
-  at resources.jobs.viaref.schedule
-  in databricks.yml:9:17
-
 {
   "pause_status": "UNPAUSED",
   "quartz_cron_expression": "0 0 * * * ?",

acceptance/bundle/validate/references/job_schedule_whole_reference/test.toml

@@ -11,6 +11,7 @@ import (
 	"github.com/databricks/cli/bundle/internal/validation/generated"
 	"github.com/databricks/cli/libs/diag"
 	"github.com/databricks/cli/libs/dyn"
+	"github.com/databricks/cli/libs/dyn/dynvar"
 	"github.com/databricks/cli/libs/structs/structpath"
 )
 
@@ -50,6 +51,12 @@ func warnForMissingFields(ctx context.Context, b *bundle.Bundle) diag.Diagnostic
 			return nil
 		}
 
+		// A pure reference (e.g. "${resources.jobs.base.schedule}") has no fields of its own;
+		// the referenced value is checked where it is defined.
+		if s, ok := v.AsString(); ok && dynvar.IsPureVariableReference(s) {
+			return nil
+		}
+
 		fields := generated.RequiredFields[pattern.String()]
 		for _, field := range fields {
 			vv := v.Get(field)

bundle/config/validate/required.go

@@ -0,0 +1,29 @@
+package validate
+
+import (
+	"testing"
+
+	"github.com/databricks/cli/bundle"
+	"github.com/databricks/cli/bundle/config"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestRequiredSkipsPureReference(t *testing.T) {
+	root, diags := config.LoadFromBytes("databricks.yml", []byte(`
+resources:
+  jobs:
+    viaref:
+      schedule: ${resources.jobs.base.schedule}
+    missing:
+      schedule:
+        timezone_id: UTC
+`))
+	require.Empty(t, diags)
+	b := &bundle.Bundle{Config: *root}
+
+	diags = Required().Apply(t.Context(), b)
+	require.Len(t, diags, 1)
+	assert.Equal(t, `required field "quartz_cron_expression" is not set`, diags[0].Summary)
+	assert.Equal(t, "resources.jobs.missing.schedule", diags[0].Paths[0].String())
+}