feat(context): enhance timeout handling, tenant isolation, and budget management

- Added timeout enforcement for token counting, scoring, and compression with detailed error handling.
- Introduced tenant isolation in context caching using project and agent identifiers.
- Enhanced budget management with stricter checks for critical context overspending and buffer limitations.
- Optimized per-context locking with cleanup to prevent memory leaks in concurrent environments.
- Updated default assembly timeout settings for improved performance and reliability.
- Improved XML escaping in Claude adapter for safety against injection attacks.
- Standardized token estimation using model-specific ratios.

backend/app/services/context/budget/allocator.py

@@ -293,14 +293,18 @@ class BudgetAllocator:
         if isinstance(context_type, ContextType):
             context_type = context_type.value
 
-        # Calculate adjustment (limited by buffer)
+        # Calculate adjustment (limited by buffer for increases, by current allocation for decreases)
         if adjustment > 0:
-            # Taking from buffer
+            # Taking from buffer - limited by available buffer
             actual_adjustment = min(adjustment, budget.buffer)
             budget.buffer -= actual_adjustment
         else:
-            # Returning to buffer
-            actual_adjustment = adjustment
+            # Returning to buffer - limited by current allocation of target type
+            current_allocation = budget.get_allocation(context_type)
+            # Can't return more than current allocation
+            actual_adjustment = max(adjustment, -current_allocation)
+            # Add returned tokens back to buffer (adjustment is negative, so subtract)
+            budget.buffer -= actual_adjustment
 
         # Apply to target type
         if context_type == "system":