fix: resolve lint and duplicate step definition issues in milestone plan navigation tests

- Fix import ordering: move pathlib before typing, add blank line before third-party imports
- Remove unnecessary 'r' mode argument from open() call (defaults to read mode)
- Remove duplicate step definition for 'the content should contain' that already exists in uko_ontology_registry_steps.py
- Keep only unique step definitions specific to milestone plan navigation

This resolves the CI failures:
- ruff I001 import block unsorted error
- ruff UP015 unnecessary mode argument error
- behave AmbiguousStep duplicate step definition error

features/steps/milestone_plan_navigation_steps.py

@@ -2,7 +2,8 @@
 
 from pathlib import Path
 from typing import Any
-from behave import given, when, then
+
+from behave import given, then, when
 
 
 @given('the specification file exists at "{filepath}"')
@@ -17,26 +18,10 @@ def step_specification_file_exists(context: Any, filepath: str) -> None:
 def step_read_specification_file(context: Any) -> None:
     """Read the specification file content."""
     spec_path = getattr(context, "spec_path", Path("docs/specification.md"))
-    with open(spec_path, "r", encoding="utf-8") as f:
+    with open(spec_path, encoding="utf-8") as f:
         context.spec_content = f.read()
 
 
-@then('the content should contain "{text}"')
-def step_content_contains(context: Any, text: str) -> None:
-    """Verify the specification contains the given text."""
-    assert hasattr(context, "spec_content"), "Specification content not loaded"
-    assert text in context.spec_content, f"Text not found in specification: {text}"
-
-
-@then('the content should NOT contain "{text}"')
-def step_content_not_contains(context: Any, text: str) -> None:
-    """Verify the specification does not contain the given text."""
-    assert hasattr(context, "spec_content"), "Specification content not loaded"
-    assert text not in context.spec_content, (
-        f"Text should not be in specification: {text}"
-    )
-
-
 @then('the content should contain a table with milestone "{milestone}"')
 def step_content_contains_milestone_in_table(context: Any, milestone: str) -> None:
     """Verify the specification contains a milestone in a table."""
@@ -48,6 +33,15 @@ def step_content_contains_milestone_in_table(context: Any, milestone: str) -> No
     )
 
 
+@then('the content should NOT contain "{text}"')
+def step_content_not_contains(context: Any, text: str) -> None:
+    """Verify the specification does not contain the given text."""
+    assert hasattr(context, "spec_content"), "Specification content not loaded"
+    assert text not in context.spec_content, (
+        f"Text should not be in specification: {text}"
+    )
+
+
 @then('the content should contain "{text}" in the milestone status')
 def step_content_contains_in_milestone_status(context: Any, text: str) -> None:
     """Verify the specification contains text in the milestone status section."""