fix: handle enterprise API errors properly to prevent KeyError crashes

When enterprise API returns 403/404, the response contains error JSON
instead of expected data structure. Code was accessing fields directly
causing KeyError → 500 Internal Server Error.

Changes:
- Add enterprise-specific error classes (EnterpriseAPIError, etc.)
- Implement centralized error validation in EnterpriseRequest.send_request()
- Extract error messages from API responses (message/error/detail fields)
- Raise domain-specific errors based on HTTP status codes
- Preserve backward compatibility with raise_for_status parameter

This prevents KeyError crashes and returns proper HTTP error codes
(403/404) instead of 500 errors.

api/services/errors/__init__.py

@@ -7,6 +7,7 @@ from . import (
     conversation,
     dataset,
     document,
+    enterprise,
     file,
     index,
     message,
@@ -21,6 +22,7 @@ __all__ = [
     "conversation",
     "dataset",
     "document",
+    "enterprise",
     "file",
     "index",
     "message",