feat(server): add v0.3 legacy compatibility for database models

[sokoliva]

Description

Implements a lazy-loading fallback mechanism to handle legacy v0.3 data stored in the database. When modern Protobuf parsing fails, it attempts to validate and convert the data using v0.3 Pydantic models and conversion utilities. This ensures backward compatibility for existing records containing string-based enums and old field structures.

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Fixes #715 🦕

[gemini-code-assist]

Summary of Changes

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This pull request introduces a critical backward compatibility layer for database models, allowing the server to seamlessly read and interpret data stored in the legacy v0.3 format. This ensures that existing records, which may contain older data structures like string-based enums, can be correctly processed by the updated system without requiring a full data migration. The changes primarily involve adding a fallback mechanism during data deserialization and providing the necessary conversion utilities.

Highlights

• Legacy Data Compatibility: Implemented a lazy-loading fallback mechanism to ensure backward compatibility with v0.3 database records. When modern Protobuf parsing fails, the system now attempts to validate and convert data using v0.3 Pydantic models and dedicated conversion utilities.
• Robust Data Retrieval: Enhanced data retrieval logic in process_result_value to gracefully handle ParseError and ValueError during Protobuf deserialization, triggering the legacy conversion process for both single items and lists of items.
• Lazy Conversion Mapping: Introduced a _get_legacy_conversions function that lazily loads a mapping between current Protobuf types (TaskStatus, Message, Artifact) and their corresponding v0.3 Pydantic models and conversion functions, optimizing performance and preventing circular dependencies.
• Comprehensive Test Coverage: Added a new detailed test case (test_get_0_3_task_detailed) to rigorously verify the correct conversion of complex v0.3 task data, including string-based enums and different field structures, into modern Protobuf models.

Changelog

• src/a2a/server/models.py
  • Added imports for Callable, ParseError, ValidationError, and v0.3 compatibility modules.
  • Modified process_result_value to include a try-except block for Protobuf parsing, falling back to legacy v0.3 Pydantic model validation and conversion on failure.
  • Implemented a _get_legacy_conversions function for lazy loading of type-to-conversion mappings for TaskStatus, Message, and Artifact.
• tests/server/tasks/test_database_task_store.py
  • Added test_get_0_3_task_detailed to verify the successful retrieval and conversion of detailed legacy v0.3 task data from the database to modern Protobuf models.

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[gemini-code-assist]

Code Review

This pull request introduces a fallback mechanism for backward compatibility with v0.3 database models. When modern Protobuf parsing fails, it attempts to deserialize data using legacy v0.3 Pydantic models and then convert them. The changes are logical and include a thorough test case to validate the new functionality. My review includes suggestions to improve type hints for increased clarity and points out potentially missing imports in the new test file.