[Task]: Database stores compatibility and migration

[ishymko]

Need to research how new version is going to behave against old data based on Pydantic types and define migration path.

Applies to push notification and task stores. Ideally new code should support reading all data and map it to proto based data model without extra migrations.

Should be possible to update parsing code to also interpret old Pydantic models from types.py (removed in 1.0-dev, should be brought back). This is going to be required for #742 as well, so some shared compat subpackage can be created similar how it was done in A2A Go.

For future migrations, we should likely add protocol_version column to explicitly specify data format. It can be a nullable column with empty interpreted as legacy 0.3 format.

[ishymko]

#754 contains compat data model and conversion which can be used here.