Add security warnings and default to DockerCommandLineCodeExecutor

[ekzhu]

This commit enhances security by:

1. Default to DockerCommandLineCodeExecutor: Added create_default_code_executor()
   function that prefers Docker when available, falling back to Local with warnings

2. LocalCommandLineCodeExecutor warnings: Added security warning when
   LocalCommandLineCodeExecutor is instantiated directly, recommending Docker instead

3. CodeExecutorAgent approval warnings: Added warning when no approval_func
   is set, encouraging human oversight for code execution

4. Centralized logic: Created shared utility in autogen_ext.code_executors
   for consistent default executor behavior across the codebase

5. Updated MagenticOne: Refactored to use the shared default executor logic
   while maintaining backward compatibility

6. Test compatibility: Updated test mocks to work with the new structure

🤖 Generated with Claude Code

Co-Authored-By: Claude noreply@anthropic.com

Why are these changes needed?

Related issue number

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[codecov]

Codecov Report

❌ Patch coverage is 71.73913% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.08%. Comparing base (6f67b95) to head (276c414).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...gen-ext/src/autogen_ext/code_executors/__init__.py	65.78%	13 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7035      +/-   ##
==========================================
+ Coverage   80.95%   81.08%   +0.13%     
==========================================
  Files         237      238       +1     
  Lines       18258    18271      +13     
==========================================
+ Hits        14780    14815      +35     
+ Misses       3478     3456      -22     

Flag	Coverage Δ
unittests	80.95% <71.73%> (+<0.01%)	⬆️

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