Remove import-time print statements from OpenWebUI engine

[Copilot]

The OpenWebUI engine was executing print statements at module import time, polluting stdout even when the engine wasn't used.

Changes:

• Moved .env file loading from module level to _load_env_file() instance method
• Moved dotenv import to module level with try-except (consistent with other optional dependencies)
• Deferred execution until __init__() call

Before:

# At module level - executes on import
try:
    from dotenv import load_dotenv
    env_path = Path(__file__).parent.parent / ".env"
    if env_path.exists():
        load_dotenv(env_path)
        print(f"[OpenWebUIEngine] Loaded environment variables from {env_path}")
except ImportError:
    print("[OpenWebUIEngine] Warning: python-dotenv not installed...")

After:

# At module level - no side effects
try:
    from dotenv import load_dotenv
    DOTENV_AVAILABLE = True
except ImportError:
    DOTENV_AVAILABLE = False

# In __init__ - executes only on instantiation
def _load_env_file(self):
    """Load environment variables from project root's .env file."""
    if not DOTENV_AVAILABLE:
        return
    env_path = Path(__file__).parent.parent / ".env"
    if env_path.exists():
        load_dotenv(env_path)

Functionality unchanged - OPENWEBUI_API_KEY still loaded from .env when engine is instantiated.

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