Generalize Continuous SystemMessage merging via model_info[“multiple_system_messages”] instead of startswith("gemini-")

[SongChiYoung]

Why are these changes needed?

The current implementation of consecutive SystemMessage merging applies only to models where model_info.family starts with "gemini-".

Since PR #6327 introduced the multiple_system_messages field in model_info, we can now generalize this logic by checking whether the field is explicitly set to False.

This change replaces the hardcoded family check with a conditional that merges consecutive SystemMessage blocks whenever multiple_system_messages is set to False.

Test cases that previously depended on the "gemini" model family have been updated to reflect this configuration flag, and renamed accordingly for clarity.

In addition, for consistency across conditional logic, a follow-up PR is planned to refactor the Claude-specific transformation condition
(currently implemented via create_args.get("model", "unknown").startswith("claude-"))
to instead use the existing is_claude().

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

All modified and coverable lines are covered by tests ✅

Project coverage is 77.94%. Comparing base (99aac24) to head (e569bf0).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6345   +/-   ##
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  Coverage   77.94%   77.94%           
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  Files         214      214           
  Lines       15355    15355           
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  Hits        11968    11968           
  Misses       3387     3387           

Flag	Coverage Δ
unittests	77.94% <100.00%> (ø)

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