test(mcp): Fix two failing integration tests and linting errors

1. Fix `test_default_beads_path_auto_detection`
    - Changed beads_path to use `Field(default_factory=_default_beads_path)` so the default is evaluated at instance
    creation time, not class definition time
    - Updated test to mock both `shutil.which` and `os.access`
2. Fix `test_init_creates_beads_directory`
    - Fixed test to pass `working_dir=temp_dir` to `BdClient` instead of using `os.chdir()`
    - The `_get_working_dir()` method checks `PWD` env var first, which isn't updated by `os.chdir()`
3. Fix minor linting errors reported by `ruff` tool
4. Update `beads` version to `0.9.6` in `uv.lock` file

MCP Server test coverage is now excellent, at 92% overall maintaining our high-standards of production level quality.

```
Name                         Stmts   Miss  Cover
------------------------------------------------
src/beads_mcp/__init__.py        1      0   100%
src/beads_mcp/__main__.py        3      3     0%
src/beads_mcp/bd_client.py     214     14    93%
src/beads_mcp/config.py         51      2    96%
src/beads_mcp/models.py         92      1    99%
src/beads_mcp/server.py         58     16    72%
src/beads_mcp/tools.py          59      0   100%
------------------------------------------------
TOTAL                          478     36    92%
```

integrations/beads-mcp/tests/test_config.py

@@ -20,8 +20,10 @@ class TestConfig:
 
         # Mock shutil.which to return a test path
         with patch("shutil.which", return_value="/usr/local/bin/bd"):
-            config = Config()
-            assert config.beads_path == "/usr/local/bin/bd"
+            # Mock os.access to say the file is executable
+            with patch("os.access", return_value=True):
+                config = Config()
+                assert config.beads_path == "/usr/local/bin/bd"
 
     def test_beads_path_from_env(self, tmp_path: Path, monkeypatch: pytest.MonkeyPatch) -> None:
         """Test that BEADS_PATH environment variable is respected."""