test: add unit tests for skills parser (#1308)

Cover parse_skill_file logic:
- Valid SKILL.md parsing with all fields
- Missing required fields (name, description) return None
- Missing/wrong filename returns None
- Optional license field handling
- Custom and default relative_path behavior
- Colons in description values
- Empty front matter handling

Co-authored-by: Willem Jiang <willem.jiang@gmail.com>

backend/tests/test_skills_parser.py

@@ -0,0 +1,98 @@
+"""Tests for skill file parser."""
+
+from pathlib import Path
+
+from deerflow.skills.parser import parse_skill_file
+
+
+def _write_skill(tmp_path: Path, content: str) -> Path:
+    """Write a SKILL.md file and return its path."""
+    skill_file = tmp_path / "SKILL.md"
+    skill_file.write_text(content, encoding="utf-8")
+    return skill_file
+
+
+class TestParseSkillFile:
+    def test_valid_skill_file(self, tmp_path):
+        skill_file = _write_skill(
+            tmp_path,
+            "---\nname: my-skill\ndescription: A test skill\nlicense: MIT\n---\n\n# My Skill\n",
+        )
+        result = parse_skill_file(skill_file, "public")
+        assert result is not None
+        assert result.name == "my-skill"
+        assert result.description == "A test skill"
+        assert result.license == "MIT"
+        assert result.category == "public"
+        assert result.enabled is True
+        assert result.skill_dir == tmp_path
+        assert result.skill_file == skill_file
+
+    def test_missing_name_returns_none(self, tmp_path):
+        skill_file = _write_skill(
+            tmp_path,
+            "---\ndescription: A test skill\n---\n\nBody\n",
+        )
+        assert parse_skill_file(skill_file, "public") is None
+
+    def test_missing_description_returns_none(self, tmp_path):
+        skill_file = _write_skill(
+            tmp_path,
+            "---\nname: my-skill\n---\n\nBody\n",
+        )
+        assert parse_skill_file(skill_file, "public") is None
+
+    def test_no_front_matter_returns_none(self, tmp_path):
+        skill_file = _write_skill(tmp_path, "# Just a markdown file\n\nNo front matter here.\n")
+        assert parse_skill_file(skill_file, "public") is None
+
+    def test_nonexistent_file_returns_none(self, tmp_path):
+        skill_file = tmp_path / "SKILL.md"
+        assert parse_skill_file(skill_file, "public") is None
+
+    def test_wrong_filename_returns_none(self, tmp_path):
+        wrong_file = tmp_path / "README.md"
+        wrong_file.write_text("---\nname: test\ndescription: test\n---\n", encoding="utf-8")
+        assert parse_skill_file(wrong_file, "public") is None
+
+    def test_optional_license_field(self, tmp_path):
+        skill_file = _write_skill(
+            tmp_path,
+            "---\nname: my-skill\ndescription: A test skill\n---\n\nBody\n",
+        )
+        result = parse_skill_file(skill_file, "custom")
+        assert result is not None
+        assert result.license is None
+        assert result.category == "custom"
+
+    def test_custom_relative_path(self, tmp_path):
+        skill_file = _write_skill(
+            tmp_path,
+            "---\nname: nested-skill\ndescription: Nested\n---\n\nBody\n",
+        )
+        rel = Path("group/nested-skill")
+        result = parse_skill_file(skill_file, "public", relative_path=rel)
+        assert result is not None
+        assert result.relative_path == rel
+
+    def test_default_relative_path_is_parent_name(self, tmp_path):
+        skill_file = _write_skill(
+            tmp_path,
+            "---\nname: my-skill\ndescription: Test\n---\n\nBody\n",
+        )
+        result = parse_skill_file(skill_file, "public")
+        assert result is not None
+        assert result.relative_path == Path(tmp_path.name)
+
+    def test_colons_in_description(self, tmp_path):
+        skill_file = _write_skill(
+            tmp_path,
+            "---\nname: my-skill\ndescription: A skill: does things\n---\n\nBody\n",
+        )
+        result = parse_skill_file(skill_file, "public")
+        assert result is not None
+        assert result.description == "A skill: does things"
+
+    def test_empty_front_matter_returns_none(self, tmp_path):
+        skill_file = _write_skill(tmp_path, "---\n\n---\n\nBody\n")
+        assert parse_skill_file(skill_file, "public") is None