fix: make session store fcntl import cross-platform compatible (#1544)

* fix: make session store fcntl import cross-platform compatible (fixes #1540)

- Add conditional fcntl import like file_memory.py already has
- Use _HAS_FCNTL flag to check fcntl availability before usage
- Windows uses msvcrt for locking, Unix uses fcntl when available
- Fixes Windows test collection failure due to missing fcntl module

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

* fix: add one-time warning for degraded file locking

Addresses CodeRabbit feedback about silent no-op locking when fcntl
is unavailable on non-Windows platforms. Adds module-level sentinel
to emit warning only once, following AGENTS.md error handling principles.

Co-authored-by: Mervin Praison <MervinPraison@users.noreply.github.com>

---------

Co-authored-by: praisonai-triage-agent[bot] <272766704+praisonai-triage-agent[bot]@users.noreply.github.com>
Co-authored-by: praisonai-triage-agent[bot] <praisonai-triage-agent[bot]@users.noreply.github.com>
Co-authored-by: Mervin Praison <MervinPraison@users.noreply.github.com>

Author: praisonai-triage-agent[bot]

src/praisonai-agents/praisonaiagents/session/store.py

@@ -5,7 +5,6 @@
 Zero dependencies beyond stdlib.
 """
 
-import fcntl
 import json
 import logging
 from praisonaiagents._logging import get_logger
@@ -14,6 +13,13 @@
 import tempfile
 import threading
 import time
+
+# fcntl is Unix-only; on Windows, use msvcrt for file locking
+if sys.platform != 'win32':
+    import fcntl
+    _HAS_FCNTL = True
+else:
+    _HAS_FCNTL = False
 from dataclasses import dataclass, field
 from datetime import datetime, timezone
 from typing import Any, Dict, List, Optional
@@ -22,6 +28,9 @@
 
 logger = get_logger(__name__)
 
+# Module-level sentinel to track if we've warned about degraded locking
+_WARNED_NO_FCNTL = False
+
 # Default session directory (uses centralized paths - DRY)
 DEFAULT_SESSION_DIR = str(get_sessions_dir())
 
@@ -150,7 +159,17 @@ def acquire(self) -> bool:
                     msvcrt.locking(self._lock_file.fileno(), msvcrt.LK_NBLCK, 1)
                 else:
                     # Unix locking
-                    fcntl.flock(self._lock_file.fileno(), fcntl.LOCK_EX | fcntl.LOCK_NB)
+                    if _HAS_FCNTL:
+                        fcntl.flock(self._lock_file.fileno(), fcntl.LOCK_EX | fcntl.LOCK_NB)
+                    else:
+                        # Warn once about degraded locking (should never happen with current platform detection)
+                        global _WARNED_NO_FCNTL
+                        if not _WARNED_NO_FCNTL:
+                            logger.warning(
+                                "File locking unavailable on this platform (no fcntl/msvcrt); "
+                                "concurrent writers may corrupt session files."
+                            )
+                            _WARNED_NO_FCNTL = True
                 return True
             except (IOError, OSError, BlockingIOError):
                 if self._lock_file:
@@ -171,7 +190,9 @@ def release(self) -> None:
                     import msvcrt
                     msvcrt.locking(self._lock_file.fileno(), msvcrt.LK_UNLCK, 1)
                 else:
-                    fcntl.flock(self._lock_file.fileno(), fcntl.LOCK_UN)
+                    if _HAS_FCNTL:
+                        fcntl.flock(self._lock_file.fileno(), fcntl.LOCK_UN)
+                    # Note: No warning needed in release() as it mirrors acquire() logic
             except (IOError, OSError):
                 pass
             finally: