Add threading.RLock to InvariantService to protect shared state
(_invariants dict, _enforcement_records list) from concurrent access
by multiple threads during parallel plan execution. Prevents
RuntimeError: dictionary changed size during iteration and data
corruption in multi-threaded environments.

Changes:
- Added self._lock = RLock() in __init__
- Wrapped all public methods (add_invariant, list_invariants,
  remove_invariant, get_effective_invariants, enforce_invariants)
  with lock acquisition via context managers
- Added helper read methods: get_enforcement_records(), get_invariant(),
  get_invariants_snapshot() -- all thread-safe
- Added BDD tests for concurrent access patterns
- Updated CHANGELOG.md and CONTRIBUTORS.md

ISSUES CLOSED: #7524

@@ -0,0 +11,4 @@
+from __future__ import annotations
+
+import threading
+from typing import cast

@@ -0,0 +14,4 @@
+from typing import cast
+
+from behave import given, then, when  # type: ignore[import-untyped]
+from behave.runner import Context

@@ -0,0 +64,4 @@
+    """Create an InvariantService populated with specific invariants for enforcement."""
+    context.service = InvariantService()
+    for i in range(count):
+        inv = context.service.add_invariant(

@@ -0,0 +100,4 @@
+# ================================================================
+
+
+@when(

@@ -0,0 +134,4 @@
+        t.join(timeout=30)
+
+
+@then("the service has no RuntimeError raised during concurrent access")

@@ -0,0 +15,4 @@
+
+  Scenario Outline: Concurrent add_invariant() must not corrupt the dict
+    Given an invariant service
+    When {n} threads concurrently add {count:d} invariants through a barrier

@@ -0,0 +41,4 @@
+
+  Scenario Outline: Concurrent remove_invariant() must not race with add
+    Given an invariant service
+    When {n} threads concurrently add invariants while {m:d} concurrent threads remove different ones through a barrier

@@ -0,0 +14,4 @@
+import threading
+
+from behave import given, then, when  # type: ignore[import-untyped]
+from behave.runner import Context

@@ -0,0 +15,4 @@
+
+  Scenario Outline: Concurrent add_invariant() must not corrupt the dict
+    Given an invariant service
+    When {n} threads concurrently add {count:d} invariants through a barrier

@@ -0,0 +41,4 @@
+
+  Scenario Outline: Concurrent remove_invariant() must not race with add
+    Given an invariant service
+    When {n} threads concurrently add invariants while {m:d} concurrent threads remove different ones through a barrier