test: cover config loading behavior (#72)

## Summary
- add dedicated unit coverage for `config.py` per #65
- cover valid TOML loading, missing/invalid config fallback, and default
copy isolation
- cover packaged default rule loading, AI rule inclusion, and shared
placeholder substitution

## Validation
- `/tmp/pyspector-pdlc048-venv/bin/python -m pytest
tests/unit/test_config.py -q`
- `/tmp/pyspector-pdlc048-venv/bin/python -m py_compile
src/pyspector/config.py tests/unit/test_config.py`
- `/tmp/pyspector-pdlc048-venv/bin/python -m ruff check
src/pyspector/config.py tests/unit/test_config.py`
- `git diff --check`

Fixes #65

Author: ynachiket

src/pyspector/config.py

@@ -1,13 +1,16 @@
 import re
+from copy import deepcopy
 from pathlib import Path
-import toml # type: ignore
-import click # type: ignore
+
+import click  # type: ignore
+import toml  # type: ignore
+
 try:
     # Python 3.9+
     import importlib.resources as pkg_resources
 except ImportError:
     # Fallback for older Python versions
-    import importlib_resources as pkg_resources # type: ignore
+    import importlib_resources as pkg_resources  # type: ignore
 
 # Sentinel placed inside any rule's `exclude_pattern` to inherit the shared
 # placeholder regex declared at [defaults].exclude_pattern_placeholder. The
@@ -42,12 +45,12 @@ def load_config(config_path: Path) -> dict:
         try:
             with config_path.open('r') as f:
                 user_config = toml.load(f).get('tool', {}).get('pyspector', {})
-                config = DEFAULT_CONFIG.copy()
+                config = deepcopy(DEFAULT_CONFIG)
                 config.update(user_config)
                 return config
         except Exception as e:
             click.echo(click.style(f"Warning: Could not parse config file '{config_path}'. Using defaults. Error: {e}", fg="yellow"))
-    return DEFAULT_CONFIG
+    return deepcopy(DEFAULT_CONFIG)
 
 def get_default_rules(ai_scan: bool = False) -> str:
     """Loads the built-in TOML rules file from package resources.

tests/unit/test_config.py

@@ -0,0 +1,94 @@
+import sys
+from pathlib import Path
+
+import toml
+
+sys.path.insert(0, str(Path(__file__).parent.parent.parent / "src"))
+
+from pyspector.config import DEFAULT_CONFIG, get_default_rules, load_config
+
+
+def test_load_config_merges_valid_pyspector_section(tmp_path):
+    config_path = tmp_path / "pyspector.toml"
+    config_path.write_text(
+        """
+[tool.pyspector]
+severity = "HIGH"
+exclude = ["custom"]
+extra_setting = "kept"
+""".strip(),
+        encoding="utf-8",
+    )
+
+    config = load_config(config_path)
+
+    assert config["severity"] == "HIGH"
+    assert config["exclude"] == ["custom"]
+    assert config["extra_setting"] == "kept"
+
+
+def test_load_config_uses_defaults_when_file_is_missing(tmp_path):
+    config = load_config(tmp_path / "missing.toml")
+
+    assert config == DEFAULT_CONFIG
+    assert config["severity"] == "LOW"
+    assert "node_modules" in config["exclude"]
+    assert "**/test_*.py" in config["exclude"]
+
+
+def test_load_config_uses_defaults_for_invalid_toml(tmp_path, capsys):
+    config_path = tmp_path / "pyspector.toml"
+    config_path.write_text("[tool.pyspector\nseverity = 'HIGH'\n", encoding="utf-8")
+
+    config = load_config(config_path)
+
+    assert config == DEFAULT_CONFIG
+    assert "Could not parse config file" in capsys.readouterr().out
+
+
+def test_load_config_defaults_are_copied_before_user_updates(tmp_path):
+    config_path = tmp_path / "pyspector.toml"
+    config_path.write_text(
+        """
+[tool.pyspector]
+severity = "MEDIUM"
+""".strip(),
+        encoding="utf-8",
+    )
+
+    config = load_config(config_path)
+    config["exclude"].append("local-only")
+
+    assert config["severity"] == "MEDIUM"
+    assert "local-only" not in DEFAULT_CONFIG["exclude"]
+
+
+def test_get_default_rules_loads_parseable_builtin_rules():
+    rules = toml.loads(get_default_rules())
+
+    rule_ids = {rule["id"] for rule in rules["rule"]}
+    assert "PY001" in rule_ids
+    assert "AI202" not in rule_ids
+
+
+def test_get_default_rules_includes_ai_rules_when_enabled(capsys):
+    rules = toml.loads(get_default_rules(ai_scan=True))
+
+    rule_ids = {rule["id"] for rule in rules["rule"]}
+    assert "PY001" in rule_ids
+    assert "AI202" in rule_ids
+    assert "AI scanning enabled" in capsys.readouterr().out
+
+
+def test_get_default_rules_substitutes_shared_placeholder_regex():
+    rules_text = get_default_rules(ai_scan=True)
+    rules = toml.loads(rules_text)
+
+    placeholder = rules["defaults"]["exclude_pattern_placeholder"]
+    assert "__SHARED_PLACEHOLDERS__" not in rules_text
+    assert placeholder
+
+    placeholder_backed_rules = [
+        rule for rule in rules["rule"] if rule.get("exclude_pattern") == placeholder
+    ]
+    assert placeholder_backed_rules