fix(sessions): use model_validate to deserialize nested EventCompaction

[hung12ct]

EventCompaction was incorrectly deserialized as dict instead of object when loading from database. Changed model_copy() to model_validate() which properly deserializes nested Pydantic models.

Fixes: AttributeError: 'dict' object has no attribute 'start_timestamp'

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Link to Issue or Description of Change

1. Link to an existing issue (if applicable): #3487

2. Or, if no issue exists, describe the change:

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Problem:
When using DatabaseSessionService (for example: with MySQL), events with EventCompaction crash after being loaded from the database with the following error:

AttributeError: 'dict' object has no attribute 'start_timestamp' 

Root Cause:

The StorageEvent.to_event() method in database_session_service.py (line 349) uses model_copy() to deserialize EventActions:

actions=EventActions().model_copy(update=self.actions.model_dump())

Issue: model_copy() performs a shallow copy and does not properly deserialize nested Pydantic models. When self.actions.model_dump() returns a dict containing compaction as a nested dict (e.g., {'compaction': {'start_timestamp': 1.0, 'end_timestamp': 2.0, ...}}), model_copy() leaves it as a dict instead of converting it to an EventCompaction object.

This causes runtime errors when code attempts to access compaction attributes like compaction.start_timestamp, because Python tries to access an attribute on a dict instead of an EventCompaction instance.

Solution:

Changed model_copy() to model_validate() which properly deserializes nested Pydantic models:

actions=EventActions.model_validate(self.actions.model_dump())

model_validate() performs full validation and deserialization, converting nested dicts into their corresponding Pydantic model instances (EventCompaction).

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• I have added tests that prove my fix is effective or that my feature works.
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[gemini-code-assist]

Summary of Changes

Hello @hung12ct, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request resolves a critical bug in the DatabaseSessionService where nested Pydantic models, specifically EventCompaction within EventActions, were not being properly deserialized from the database. The previous use of model_copy() resulted in these nested models being treated as plain dictionaries, causing runtime AttributeError when attempting to access their attributes. By switching to model_validate(), the system now correctly reconstructs the full Pydantic object hierarchy, ensuring data integrity and preventing application crashes related to session loading.

Highlights

• Deserialization Fix for Nested Pydantic Models: Corrected an issue where EventCompaction within EventActions was incorrectly deserialized as a dictionary instead of a Pydantic object when loaded from the database, leading to AttributeError.
• Transition from model_copy() to model_validate(): The StorageEvent.to_event() method in database_session_service.py now uses EventActions.model_validate() instead of EventActions().model_copy() to ensure proper, deep deserialization of nested Pydantic models.
• New Comprehensive Test Suite: A new test file, test_database_compaction_deserialization.py, has been added to verify that EventCompaction objects retain their type and attributes after being saved to and loaded from various session services (In-Memory, Database, Sqlite).

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[gemini-code-assist]

Code Review

This pull request addresses a deserialization issue with nested EventCompaction objects within EventActions when using DatabaseSessionService. The fix correctly replaces model_copy() with model_validate(), ensuring that nested Pydantic models are properly instantiated from dictionary representations, which resolves the AttributeError. The change is accompanied by a new, well-structured test file that provides excellent coverage for this fix. The tests are parameterized to run against multiple session service implementations, verifying the fix and checking for regressions. The overall changes are of high quality and improve the robustness of session handling.