[BUGFIX] Add LLMCallEventMessage to resolve instantiation error in AgentStudio

[SongChiYoung]

Why are these changes needed?

This PR defines the missing LLMCallEventMessage class to resolve an instantiation error that occurs when using custom messages (e.g., via AgentStudio).

Discord Report
சravanaன் — 오후 6:40
“i updated agentchat and agentcore and tried running the config from agentstudio and it is now not running the agent and is throwing error TypeError: Can't instantiate abstract class LLMCallEventMessage with abstract methods to_model_message, to_model_text, to_text”

The issue stems from LLMCallEventMessage being an abstract class that lacks required methods from BaseChatMessage.
This PR implements the missing methods.

Since LLMCallEventMessage is intended for logging/UI use only, and not to be sent to LLMs, the to_model_message() method raises NotImplementedError by design.

Related issue number

Reported in Discord
Closes #6206

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 77.24%. Comparing base (b62b12e) to head (d02d6d0).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6204   +/-   ##
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  Coverage   77.24%   77.24%           
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  Files         200      200           
  Lines       14212    14212           
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  Hits        10978    10978           
  Misses       3234     3234           

Flag	Coverage Δ
unittests	77.24% <ø> (ø)

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