fix: Failed to call _from_config() when HttpTool in StaticStreamWorkbench

[wooyongbin3-cpu]

Auto-generated fix for #7172

What happened?

Describe the bug
I created a tool using HttpTool and used it in StaticStreamWorkbench. It worked fine.
However, when I created a new instance of StaticStreamWorkbench by calling _from_config(), I got an error:

pydantic_core._pydantic_core.ValidationError: 1 validation error for HttpToolConfig
headers
  Field required [type=missing, input_value={'name': 'base64_decode',... 'text', 'timeout': 5.0}, input_type=dict]
    For further information visit https://errors.pydantic.dev/2.12/v/missing

Maybe HttpTool has some problems to cause the failure of _from_config().

env version:

• autogen-core: 0.7.5
• autogen-ext: 0.7.5
• Python 3.13.9

To Reproduce
Here is the code:

from autogen_core.tools import StaticStreamWorkbench
from autogen_ext.tools.h

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