feat(platform): add custom exception hierarchy (GAP-7)

Add PlatformError base class and 5 specific subclasses:
- NotFoundError (404) - resource not found
- DuplicateError (409) - resource already exists
- AuthenticationError (401) - invalid credentials
- AuthorizationError (403) - insufficient permissions
- ValidationError (422) - invalid input data

Each exception includes default message and accepts custom messages.
Importable from praisonai_platform.exceptions.

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

Author: praisonai-triage-agent[bot]

src/praisonai-platform/praisonai_platform/exceptions.py

@@ -0,0 +1,44 @@
+"""Custom exception hierarchy for the platform package."""
+
+
+class PlatformError(Exception):
+    """Base exception for all platform errors."""
+    
+    def __init__(self, message: str = "A platform error occurred"):
+        self.message = message
+        super().__init__(message)
+
+
+class NotFoundError(PlatformError):
+    """Raised when a requested resource is not found (maps to HTTP 404)."""
+    
+    def __init__(self, message: str = "Resource not found"):
+        super().__init__(message)
+
+
+class DuplicateError(PlatformError):
+    """Raised when a resource already exists (maps to HTTP 409)."""
+    
+    def __init__(self, message: str = "Resource already exists"):
+        super().__init__(message)
+
+
+class AuthenticationError(PlatformError):
+    """Raised when authentication credentials are invalid (maps to HTTP 401)."""
+    
+    def __init__(self, message: str = "Authentication failed"):
+        super().__init__(message)
+
+
+class AuthorizationError(PlatformError):
+    """Raised when user lacks sufficient permissions (maps to HTTP 403)."""
+    
+    def __init__(self, message: str = "Insufficient permissions"):
+        super().__init__(message)
+
+
+class ValidationError(PlatformError):
+    """Raised when input data is invalid (maps to HTTP 422)."""
+    
+    def __init__(self, message: str = "Invalid input data"):
+        super().__init__(message)