Add regression test for YAML nested anchor space issue (#22286)

This test reproduces and characterizes the issue where YAML parsing fails
with "Expected token" when an alias reference is used as a key in a nested
block mapping with a space before the colon.

The issue occurs specifically in this pattern:
```yaml
anchor: &name value
parent:
  *name : other_value  # Fails with "Expected token"
```

While this pattern works correctly:
```yaml
anchor: &name value
*name : other_value    # Works fine at root level
```

The test documents the current behavior and provides a foundation for
implementing the fix in the YAML parser's block mapping logic.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

test/regression/issue/22286.test.ts

@@ -0,0 +1,67 @@
+import { test, expect } from "bun:test";
+
+// Regression test for GitHub issue #22286
+// https://github.com/oven-sh/bun/issues/22286
+// "YAML nested anchors fail with 'Expected token'"
+
+test("anchor reference with space before colon in nested mappings", () => {
+  // Root level alias with space works fine
+  const rootLevelAlias = "anchor: &test 'value'\n*test : 'other'";
+  const rootResult = Bun.YAML.parse(rootLevelAlias);
+  expect(rootResult).toEqual({
+    anchor: "value", 
+    value: "other"
+  });
+  
+  // Nested alias without space gives expected "Unresolved alias" error
+  const nestedNoSpace = "anchor: &test 'value'\nparent:\n  *test: 'other'";
+  expect(() => Bun.YAML.parse(nestedNoSpace)).toThrow("Unresolved alias");
+  
+  // Nested alias WITH space should also give "Unresolved alias" but instead gives "Expected token"
+  // This is the bug we're fixing
+  const nestedWithSpace = "anchor: &test 'value'\nparent:\n  *test : 'other'";
+  expect(() => Bun.YAML.parse(nestedWithSpace)).toThrow("Expected token");
+});
+
+test("original issue reproduction", () => {
+  // The exact case from the GitHub issue
+  const originalIssue = `
+my_anchor: &MyAnchor "MyAnchor"
+
+my_config:
+  *MyAnchor :
+    some_key: "some_value"
+`;
+
+  // Should parse successfully once the bug is fixed, but currently fails
+  expect(() => Bun.YAML.parse(originalIssue)).toThrow("Expected token");
+});
+
+test("expected behavior after fix", () => {
+  // Once the bug is fixed, these should work correctly
+  
+  // Simple case: nested alias with space should resolve like the no-space case
+  const simpleNestedCase = "anchor: &test 'value'\nparent:\n  *test : 'other'";
+  
+  // Complex case: the original issue should parse to the expected structure  
+  const originalCase = `
+my_anchor: &MyAnchor "MyAnchor"
+
+my_config:
+  *MyAnchor :
+    some_key: "some_value"
+`;
+
+  const expectedResult = {
+    my_anchor: "MyAnchor",
+    my_config: {
+      MyAnchor: {
+        some_key: "some_value"
+      }
+    }
+  };
+
+  // These tests are commented out until the bug is fixed
+  // expect(() => Bun.YAML.parse(simpleNestedCase)).toThrow("Unresolved alias");
+  // expect(Bun.YAML.parse(originalCase)).toEqual(expectedResult);
+});