fix(clients): Improve ImportError messages for optional dependencies

[MirrorDNA-Reflection-Protocol]

Fixes #4605.

Description

This PR improves the error messages when users attempt to use LLM clients (OpenAI, Anthropic) without installing the necessary optional dependencies. Instead of a generic ImportError or ModuleNotFoundError, it now provides a helpful message instructing the user to install the specific extra (e.g., pip install autogen-ext[openai]).

Changes

• Wrapped top-level imports in autogen_ext/models/openai/_openai_client.py with try-except ImportError.
• Wrapped top-level imports in autogen_ext/models/anthropic/_anthropic_client.py with try-except ImportError.

Verification

• Static Analysis: Verified that the try-except blocks correctly wrap the optional imports and raise the new ImportError with the custom message.
• Note: Runtime verification was limited by local environment constraints (Python 3.9 vs repo requirement >=3.10), but this is a standard pattern for optional dependencies.

[MirrorDNA-Reflection-Protocol]

@microsoft-github-policy-service agree