Show an error when non-yaml files used in include section (#2201)

## Changes
`include` section is used only to include other bundle configuration
YAML files. If any other file type is used, raise an error and guide
users to use `sync.include` instead

## Tests
Added acceptance test

---------

Co-authored-by: Julia Crawford (Databricks) <julia.crawford@databricks.com>

acceptance/bundle/includes/non_yaml_in_include/databricks.yml

@@ -0,0 +1,6 @@
+bundle:
+  name: non_yaml_in_includes
+
+include:
+ - test.py
+ - resources/*.yml

acceptance/bundle/includes/non_yaml_in_include/output.txt

@@ -0,0 +1,10 @@
+Error: Files in the 'include' configuration section must be YAML files.
+  in databricks.yml:5:4
+
+The file test.py in the 'include' configuration section is not a YAML file, and only YAML files are supported. To include files to sync, specify them in the 'sync.include' configuration section instead.
+
+Name: non_yaml_in_includes
+
+Found 1 error
+
+Exit code: 1

acceptance/bundle/includes/non_yaml_in_include/script

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

acceptance/bundle/includes/non_yaml_in_include/test.py

@@ -0,0 +1 @@
+print("Hello world")

bundle/config/loader/process_root_includes.go

@@ -2,6 +2,7 @@ package loader
 
 import (
 	"context"
+	"fmt"
 	"path/filepath"
 	"slices"
 	"strings"
@@ -36,6 +37,7 @@ func (m *processRootIncludes) Apply(ctx context.Context, b *bundle.Bundle) diag.
 	// Maintain list of files in order of files being loaded.
 	// This is stored in the bundle configuration for observability.
 	var files []string
+	var diags diag.Diagnostics
 
 	// For each glob, find all files to load.
 	// Ordering of the list of globs is maintained in the output.
@@ -60,7 +62,7 @@ func (m *processRootIncludes) Apply(ctx context.Context, b *bundle.Bundle) diag.
 
 		// Filter matches to ones we haven't seen yet.
 		var includes []string
-		for _, match := range matches {
+		for i, match := range matches {
 			rel, err := filepath.Rel(b.BundleRootPath, match)
 			if err != nil {
 				return diag.FromErr(err)
@@ -69,9 +71,22 @@ func (m *processRootIncludes) Apply(ctx context.Context, b *bundle.Bundle) diag.
 				continue
 			}
 			seen[rel] = true
+			if filepath.Ext(rel) != ".yaml" && filepath.Ext(rel) != ".yml" {
+				diags = diags.Append(diag.Diagnostic{
+					Severity:  diag.Error,
+					Summary:   "Files in the 'include' configuration section must be YAML files.",
+					Detail:    fmt.Sprintf("The file %s in the 'include' configuration section is not a YAML file, and only YAML files are supported. To include files to sync, specify them in the 'sync.include' configuration section instead.", rel),
+					Locations: b.Config.GetLocations(fmt.Sprintf("include[%d]", i)),
+				})
+				continue
+			}
 			includes = append(includes, rel)
 		}
 
+		if len(diags) > 0 {
+			return diags
+		}
+
 		// Add matches to list of mutators to return.
 		slices.Sort(includes)
 		files = append(files, includes...)