perf: decrease mem cost

[fatelei]

Description

fix #756

previously, ModelProperties was converted inside every MarshalJSON call:

result := mapping.ConvertAnyMap(m.ModelProperties)
m.ModelProperties = result

However, because MarshalJSON was defined on a value receiver, m is a copy, so the converted result is never written back to the original object. The next serialization still repeats the conversion from scratch.
Now this is done early, after deserialization/loading, using a pointer receiver:

m.ModelProperties = result

The original object is normalized in place, so subsequent MarshalJSON calls serialize the already-converted map[string]any directly without repeating the recursive conversion.

Type of Change

• Bug fix
• New feature
• Refactor
• Performance improvement
• Other

Essential Checklist

Testing

• I have tested the changes locally and confirmed they work as expected
• I have added unit tests where necessary and they pass successfully

Bug Fix (if applicable)

• I have used GitHub syntax to close the related issue (e.g., Fixes #123 or Closes #123)

Additional Information

Please provide any additional context that would help reviewers understand the changes.

[gemini-code-assist]

Code Review

This pull request refactors session message serialization by introducing a structured pluginInStreamMessage type and using standard json.Marshal instead of a custom parser. It also moves the normalization of model properties from serialization (MarshalJSON) to deserialization (UnmarshalJSON/UnmarshalYAML) and cache retrieval steps, accompanied by new unit tests. The review feedback highlights a potential issue in Session.Message where the error from json.Marshal is ignored, which could lead to silent failures or panics if serialization fails, and suggests logging this error.

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