Support YAML anchors and aliases (#234)

Fixes https://github.com/github/vscode-github-actions/issues/405

YAML anchors (&name) and aliases (*name) are now properly supported.
When an alias is encountered during parsing, it is resolved to its
anchored value, making aliases transparent to the rest of the system.

Changes:
- workflow-parser: Handle isAlias nodes in YamlObjectReader.getNodes()
- languageservice: Add tests for various anchor/alias patterns

Test cases:
- Anchors in env mappings
- Multiple aliases to same anchor
- Anchors in matrix strategy
- Anchors in steps
- Scalar anchors (e.g., runs-on)

languageservice/src/validate.yaml-anchors.test.ts

@@ -0,0 +1,202 @@
+import {validate} from "./validate";
+import {createDocument} from "./test-utils/document";
+import {clearCache} from "./utils/workflow-cache";
+
+beforeEach(() => {
+  clearCache();
+});
+
+describe("YAML anchors and aliases", () => {
+  it("should handle anchors and aliases in env", async () => {
+    const doc = createDocument(
+      "wf.yaml",
+      `
+on: push
+jobs:
+  job1:
+    runs-on: ubuntu-latest
+    env: &env
+      ENV1: env1
+      ENV2: env2
+    steps:
+      - run: exit 0
+  job2:
+    runs-on: ubuntu-latest
+    env: *env
+    steps:
+      - run: exit 0
+`
+    );
+    const result = await validate(doc);
+    expect(result).toEqual([]);
+  });
+
+  it("should handle multiple aliases to the same anchor", async () => {
+    const doc = createDocument(
+      "wf.yaml",
+      `
+on: push
+env: &shared
+  SHARED: true
+jobs:
+  job1:
+    runs-on: ubuntu-latest
+    env: *shared
+    steps:
+      - run: exit 0
+  job2:
+    runs-on: ubuntu-latest
+    env: *shared
+    steps:
+      - run: exit 0
+  job3:
+    runs-on: ubuntu-latest
+    env: *shared
+    steps:
+      - run: exit 0
+`
+    );
+    const result = await validate(doc);
+    expect(result).toEqual([]);
+  });
+
+  it("should handle anchors in matrix strategy", async () => {
+    const doc = createDocument(
+      "wf.yaml",
+      `
+on: push
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        include: &matrix-include
+          - os: ubuntu-latest
+            node: 18
+          - os: windows-latest
+            node: 20
+    steps:
+      - run: exit 0
+  test2:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        include: *matrix-include
+    steps:
+      - run: exit 0
+`
+    );
+    const result = await validate(doc);
+    expect(result).toEqual([]);
+  });
+
+  it("should handle anchors in steps", async () => {
+    const doc = createDocument(
+      "wf.yaml",
+      `
+on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - &checkout
+        uses: actions/checkout@v4
+      - run: npm test
+  deploy:
+    runs-on: ubuntu-latest
+    steps:
+      - *checkout
+      - run: npm run deploy
+`
+    );
+    const result = await validate(doc);
+    expect(result).toEqual([]);
+  });
+
+  it("should handle scalar anchors", async () => {
+    const doc = createDocument(
+      "wf.yaml",
+      `
+on: push
+jobs:
+  build:
+    runs-on: &runner ubuntu-latest
+    steps:
+      - run: exit 0
+  test:
+    runs-on: *runner
+    steps:
+      - run: exit 0
+`
+    );
+    const result = await validate(doc);
+    expect(result).toEqual([]);
+  });
+
+  it("should work without anchors (control test)", async () => {
+    const doc = createDocument(
+      "wf.yaml",
+      `
+on: push
+jobs:
+  job1:
+    runs-on: ubuntu-latest
+    env:
+      ENV1: env1
+      ENV2: env2
+    steps:
+      - run: exit 0
+  job2:
+    runs-on: ubuntu-latest
+    env:
+      ENV1: env1
+      ENV2: env2
+    steps:
+      - run: exit 0
+`
+    );
+    const result = await validate(doc);
+    expect(result).toEqual([]);
+  });
+
+  it("should handle circular aliases without hanging", async () => {
+    // This is an invalid use case (alias referencing parent) but should not hang
+    const doc = createDocument(
+      "wf.yaml",
+      `
+on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    env: &myenv
+      FOO: bar
+      nested: *myenv
+    steps:
+      - run: exit 0
+`
+    );
+    // Should complete without hanging - circular portion is silently ignored
+    // which may cause downstream validation errors, but that's acceptable
+    const result = await validate(doc);
+    expect(result).toBeDefined();
+  });
+
+  it("should handle undefined alias references", async () => {
+    // Reference to non-existent anchor - yaml library should report error
+    const doc = createDocument(
+      "wf.yaml",
+      `
+on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    env: *nonexistent
+    steps:
+      - run: exit 0
+`
+    );
+    const result = await validate(doc);
+    expect(result).toBeDefined();
+    expect(result.length).toBeGreaterThan(0);
+  });
+});