[protobuf-schema] Remove 'model/' prefix from Protobuf imports for mapped custom packages

[lucy66hw]

This PR fixes the issue by removing the "model/" prefix from imports that are explicitly mapped through importMapping. The generator now correctly handles standard protobuf types and ensures generated files compile successfully.

Example of Incorrect Import:
import "model/google/protobuf/struct.proto";
Now:
import "google/protobuf/struct.proto";

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