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 +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 +55,4 @@
+    Given an invariant service with {count:d} invariants to enforce
+    When {n:d} threads concurrently call enforce_invariants on the same set through a barrier
+    Then no thread raised RuntimeError during enforcement
+    And each thread should have received exactly {total:d} enforcement records

@@ -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

@@ -0,0 +55,4 @@
+    Given an invariant service with {count:d} invariants to enforce
+    When {n:d} threads concurrently call enforce_invariants on the same set through a barrier
+    Then no thread raised RuntimeError during enforcement
+    And each thread should have received exactly {total:d} enforcement records

@@ -0,0 +118,4 @@
+                    source_name="concurrent-test",
+                )
+        except Exception as exc:
+            with context._error_mutex if hasattr(context, "_error_mutex") else threading.Lock():

@@ -0,0 +209,4 @@
+    )
+
+
+@then("the caller should receive a valid list of at least {min:d} active invariants")