FIX: GraphFlow serialize/deserialize and adding test

[SongChiYoung]

Why are these changes needed?

❗ Before

Previously, GraphFlow.init() modified the inner_chats and termination_condition for internal execution logic (e.g., constructing _StopAgent or composing OrTerminationCondition).
However, these modified values were also used during dump_component(), meaning the serialized config no longer matched the original inputs.

As a result:
1. dump_component() → load_component() → dump_component() produced non-idempotent configs.
2. Internal-only constructs like _StopAgent were mistakenly serialized, even though they should only exist in runtime.

⸻

✅ After

This patch changes the behavior to:
• Store original inner_chats and termination_condition as-is at initialization.
• During to_config(), serialize only the original unmodified versions.
• Avoid serializing _StopAgent or other dynamically built agents.
• Ensure deserialization (from_config) produces a logically equivalent object without additional nesting or duplication.

This ensures that:
• GraphFlow.dump_component() → load_component() round-trip produces consistent, minimal configs.
• Internal execution logic and serialized component structure are properly separated.

Related issue number

Closes #6431

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.

[ekzhu]

@abhinav-aegis for awareness

[ekzhu]

Thanks for the fix!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.55%. Comparing base (8efa1f1) to head (cbdbeb0).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6434      +/-   ##
==========================================
+ Coverage   78.51%   78.55%   +0.03%     
==========================================
  Files         225      225              
  Lines       16521    16523       +2     
==========================================
+ Hits        12971    12979       +8     
+ Misses       3550     3544       -6     

Flag	Coverage Δ
unittests	78.55% <100.00%> (+0.03%)	⬆️

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[abhinav-aegis]

Thx!