fix: serialize Pydantic models before storing in session state

[ayushozha]

Summary

• Call .model_dump() on Pydantic models (IntentMandate, CartMandate, PaymentMandate, ContactAddress, PaymentReceipt) before storing them in ADK session state, ensuring they are JSON-serializable dicts
• Reconstruct Pydantic models from dicts when reading them back from state where attribute access is needed (e.g., cart_mandate.contents.payment_request)
• This fixes the TypeError: Object of type IntentMandate is not JSON serializable error when using sqlite_session_service

Test plan

• Run the shopping agent end-to-end flow with sqlite_session_service and verify no JSON serialization errors
• Verify intent mandate creation and product search still works
• Verify cart selection, update, and payment mandate creation flow works
• Verify payment initiation (with and without OTP) works
• Verify the credentials provider interactions still work

Fixes #129

🤖 Generated with Claude Code

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[gemini-code-assist]

Summary of Changes

Hello, 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 addresses a critical serialization issue within the shopping agent's session state management. By ensuring that Pydantic models are converted to JSON-serializable dictionaries before storage and reconstructed upon retrieval, the changes enable robust operation with session services like sqlite_session_service, preventing runtime errors and ensuring the agent's end-to-end flows function correctly.

Highlights

• Pydantic Model Serialization: Implemented serialization of Pydantic models (IntentMandate, CartMandate, PaymentMandate, ContactAddress, PaymentReceipt) using .model_dump() before storing them in ADK session state. This ensures they are JSON-serializable dictionaries.
• Pydantic Model Deserialization: Added logic to reconstruct Pydantic models from their dictionary representations when retrieving them from session state, allowing for proper attribute access.
• Bug Fix: Resolved the TypeError: Object of type IntentMandate is not JSON serializable error that occurred when using sqlite_session_service.

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[gemini-code-assist]

Code Review

This pull request effectively resolves the JSON serialization error by consistently using .model_dump() when storing Pydantic models in the session state and reconstructing them when they are read back and need to be accessed as objects. The changes are logical and well-implemented across the affected files. I have one minor suggestion to improve code clarity and consistency regarding variable naming.

[gemini-code-assist]

This change is correct since intent_mandate is now a dictionary. For better clarity and consistency with other parts of this PR (e.g., update_chosen_cart_mandate where cart_mandate_dicts is used), consider renaming the intent_mandate variable to intent_mandate_dict on line 87. This would make it explicit that it's a dictionary and not a Pydantic model instance, improving code readability.