feat(#868k5bbdr): Add pickle-safe enum + pydantic-core schema validation

[vavsab]

Cherry-picks the two behavioral patches that make the mapped/betterproto Python package a drop-in replacement for the jbkoh/python-betterproto@844ab9d git URL that mappedgpt-bot / mapped2 currently pin.

Without these, generated pydantic models silently fall back to plain int_schema (losing enum-instance validation) and enum values can't round-trip through pickle.

Patches (both authored by Jason Koh):

• migrate the enum serialization (987eec6): change generated __get_pydantic_core_schema__ from plain int_schema to a chain that validates and reconstructs the enum instance
• proper unpickle for enums (844ab9d): add __reduce__ on the base Enum class

Follow-up: after this merges and publishes as e.g. betterproto-1.0.2 on fury, mappedgpt-bot switches from the git URL to betterproto = {index = "fury"} and drops apt-get install git from the Dockerfile.