Improve MCP startup errors for missing server dependencies

[Rishirandhawa]

Summary

• add MCP startup error formatting in McpWorkbench.list_tools() to surface actionable guidance instead of opaque actor crashes
• include specific Playwright MCP install guidance when the configured stdio command targets @playwright/mcp
• include command-not-found guidance for missing stdio executables
• add tests covering both new startup error paths

Why

AssistantAgent MCP workflows often fail with generic connection errors when MCP server dependencies are missing. This makes the README quickstart hard to debug, especially for Playwright MCP setup.

Fixes #7337

Validation

• python3 -m ruff check python/packages/autogen-ext/src/autogen_ext/tools/mcp/_workbench.py python/packages/autogen-ext/tests/tools/test_mcp_workbench_warnings_and_errors.py
• uv run --package autogen-ext --extra mcp pytest -q packages/autogen-ext/tests/tools/test_mcp_workbench_warnings_and_errors.py -k "playwright_dependency_hint or missing_stdio_command or list_tools_actor_none_after_start"

[Rishirandhawa]

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[Rishirandhawa]

@microsoft-github-policy-service agree