construct a better exc message when MCP call fails

[peterj]

Why are these changes needed?

This improves the message in case there's an exception when running an MCP tool.

I've been seeing errors like this one:

unhandled errors in a TaskGroup (1 sub-exception)

With this change, the error includes the actual details from all exceptions in the exception group. For example

ExceptionGroup: unhandled errors in a TaskGroup (1 sub-exception)
('unhandled errors in a TaskGroup', [Exception("MCP tool execution failed: [TextContent(type='text', text='Place search failed: This API project is not authorized to use this API.', annotations=None)]")])

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

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 75.53%. Comparing base (cf13657) to head (43c67bd).
Report is 66 commits behind head on main.

Files with missing lines	Patch %	Lines
...ges/autogen-ext/src/autogen_ext/tools/mcp/_base.py	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5900      +/-   ##
==========================================
+ Coverage   70.15%   75.53%   +5.37%     
==========================================
  Files         263      191      -72     
  Lines       15122    13087    -2035     
  Branches      256        0     -256     
==========================================
- Hits        10609     9885     -724     
+ Misses       4313     3202    -1111     
+ Partials      200        0     -200     

Flag	Coverage Δ
unittests	75.53% <0.00%> (+5.37%)	⬆️

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[ekzhu]

You can follow the Python developer guide to run checks locally. https://github.com/microsoft/autogen/blob/13804125823649f2d199c5127dd9cbffdae4d622/python/README.md

[ekzhu]

This is resolved by #6125