fix: improve import error messages by chaining original exception (#4605)

[ddarji1409]

Summary

Fixes #4605 — improves import error diagnostics in autogen-ext by preserving the original exception rather than suppressing it.

Changes:

• tools/azure/_ai_search.py: capture ImportError as e and raise from e (instead of from None) for both azure_openai and openai embedding provider paths; also includes the original error text in the message so users see the root cause (e.g. missing sub-dependency like azure-identity)
• memory/canvas/_text_canvas.py: store the ModuleNotFoundError at import time and chain it when the deferred ImportError is raised at the call site

Before:

ImportError: Azure OpenAI SDK is required for client-side embedding generation. Please install it with: uv add openai azure-identity

After:

ImportError: Azure OpenAI SDK is required for client-side embedding generation. Original error: No module named 'azure.identity'
Please install it with: uv add openai azure-identity

Test plan

• Uninstall openai and verify the error message includes the original ModuleNotFoundError text
• Uninstall azure-identity and verify the error message shows the specific missing module
• Uninstall unidiff and call apply_patch() — verify chained exception is visible in traceback

🤖 Generated with Claude Code

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