Accept inline YAML merge mappings in bundles (#7269)

**Bundles using an inline YAML merge mapping now pass validation.** The
[merged acceptance
fixture](https://github.com/databricks/cli/blob/a7e611477d067181ea538d3d8e1bd44ec1d9dfb5/acceptance/bundle/validate/yaml_anchors/inline_merge_mapping/test.toml)
records `<<: {x: overridden}` as valid input that `bundle validate`
previously rejected with `map merge requires map or sequence of maps as
the value`. [PR #7085](#7085)
introduced this fixture to pin that behavior.

The YAML loader now accepts a mapping node as one merge source. The
updated acceptance case checks `bundle validate -t dev` without
`--strict` and the resolved JSON value of `variables.x`; a focused unit
test checks that an explicit key takes precedence while another merged
key remains.

## Local verification

The five-file diff was checked on
`codex/dabs-yaml-inline-merge-20261030` at its [recorded upstream
base](a7e6114):

- `TestYAMLInlineMapMerge` passed.
- The focused inline-merge acceptance case passed without `--strict`
with `DMS=` and `DMS=true`. Both modes reported `Validation OK!` and
JSON `default` and `value` of `overridden`.
- `git diff --check` passed.

These are local checks; this body makes no CI claim.

## Candidate assignee

Denis Bilenko (`denik`) authored [PR
#7085](#7085), which added the
inline-merge fixture.

Author: lennartkats-db

acceptance/bundle/validate/yaml_anchors/inline_merge_mapping/output.txt

@@ -1,8 +1,15 @@
 
->>> [CLI] bundle validate
-Error: failed to load [TEST_TMP_DIR]/databricks.yml: yaml ([TEST_TMP_DIR]/databricks.yml:11:11): map merge requires map or sequence of maps as the value
-
-
-Found 1 error
-
-Exit code: 1
+>>> [CLI] bundle validate -t dev
+Name: test-bundle
+Target: dev
+Workspace:
+  User: [USERNAME]
+  Path: /Workspace/Users/[USERNAME]/.bundle/test-bundle/dev
+
+Validation OK!
+
+>>> [CLI] bundle validate -t dev -o json
+{
+  "default": "overridden",
+  "value": "overridden"
+}

acceptance/bundle/validate/yaml_anchors/inline_merge_mapping/script

@@ -1,2 +1,3 @@
 # An inline mapping as the value of '<<' in target variables.
-errcode trace $CLI bundle validate
+trace $CLI bundle validate -t dev
+trace $CLI bundle validate -t dev -o json | jq .variables.x

acceptance/bundle/validate/yaml_anchors/inline_merge_mapping/test.toml

@@ -169,7 +169,7 @@ func (d *loader) loadMapping(node *yaml.Node, loc diag.Location) (dyn.Value, err
 	switch merge.Kind {
 	case yaml.SequenceNode:
 		mnodes = merge.Content
-	case yaml.AliasNode:
+	case yaml.MappingNode, yaml.AliasNode:
 		mnodes = []*yaml.Node{merge}
 	default:
 		return dyn.NilValue, merr

libs/dyn/yamlloader/loader.go

@@ -138,3 +138,12 @@ func TestYAMLAnchor10(t *testing.T) {
 	assert.Equal(t, 1, self.Get("use1").Get("value").AsAny())
 	assert.Equal(t, 1, self.Get("use2").Get("value").AsAny())
 }
+
+func TestYAMLInlineMapMerge(t *testing.T) {
+	input := "target:\n  <<: {x: inherited, y: merged}\n  x: local\n"
+	value, err := yamlloader.LoadYAML("inline.yml", bytes.NewBufferString(input))
+	require.NoError(t, err)
+
+	assert.Equal(t, "local", value.Get("target").Get("x").AsAny())
+	assert.Equal(t, "merged", value.Get("target").Get("y").AsAny())
+}