fix: resolve backend variable shadowing in Agent initialization

The issue was caused by a local variable 'backend' in the memory configuration
processing that was shadowing the 'backend' parameter from the Agent constructor.

When memory=True was passed to the Agent, the memory backend string (e.g., 'file')
was incorrectly assigned to self.backend instead of the external managed backend.

This caused a RuntimeError: Backend str does not support execute() method when
the Agent tried to delegate to what it thought was a managed backend but was
actually a string.

Changes:
- Rename local variable from 'backend' to 'memory_backend' to avoid shadowing
- Preserves the original backend parameter functionality
- Fixes the issue where memory=True would break external backend delegation

Fixes #1371

Co-authored-by: praisonai-triage-agent[bot] <praisonai-triage-agent[bot]@users.noreply.github.com>

Author: praisonai-triage-agent[bot]

src/praisonai-agents/praisonaiagents/agent/agent.py

@@ -1042,18 +1042,18 @@ def __init__(
                 if _memory_config.auto_save is not None:
                     auto_save = _memory_config.auto_save
                 # Convert to internal format
-                backend = _memory_config.backend
-                if hasattr(backend, 'value'):
-                    backend = backend.value
+                memory_backend = _memory_config.backend
+                if hasattr(memory_backend, 'value'):
+                    memory_backend = memory_backend.value
                 # If learn is enabled, pass as dict to trigger full Memory class
                 if _memory_config.learn:
                     memory = _memory_config.to_dict()
-                elif backend == "file":
+                elif memory_backend == "file":
                     memory = True
                 elif _memory_config.config:
                     memory = _memory_config.config
                 else:
-                    memory = backend
+                    memory = memory_backend
             elif hasattr(_memory_config, 'search') and hasattr(_memory_config, 'add'):
                 # Memory instance - pass through
                 pass