Incorrect Import Paths for Nested Message Classes

[aniketpalu]

Describe the bug
When protobuf messages define nested message types (like DataSource.FileOptions), the generated code imports and references them as flat names rather than as nested classes.

Why this is a problem
Breaks namespace organization
Causes naming conflicts when multiple messages have nested types with the same name
Requires extensive import rewrites

Dependencies

python version: sys.version_info(major=3, minor=11, micro=8, releaselevel='final', serial=0)

############# dependencies ############## 
    grpc:            1.62.3
    pydantic:        2.10.6

########## Expand dependencies ########## 
    mypy-protobuf:   3.3.0
    toml:            0.10.2

########## Format dependencies ########## 
    autoflake:       Not Install
    black:           Not Install
    isort:           Not Install

Protobuf File Content

syntax = "proto3";

message DataSource {
  message FileOptions {
    string path = 1;
  }
  
  message BigQueryOptions {
    string table = 1;
  }
  
  oneof options {
    FileOptions file = 10;
    BigQueryOptions bigquery = 11;
  }
}

message SavedDataset {
  DataSource.FileOptions file_storage = 1;
  DataSource.BigQueryOptions bigquery_storage = 2;
}

CLI(if use plugin mode)

python -m grpc_tools.protoc \
-I. \
--protobuf-to-pydantic_out=. \
feast/core/DataSource.proto feast/core/SavedDataset.proto

Output content

from .DataSource_p2p import BigQueryOptions

class SavedDataset(BaseModel):
    file_storage: FileOptions = Field(default_factory=FileOptions)
    bigquery_storage: BigQueryOptions = Field(default_factory=BigQueryOptions)

Expected behavior

from .DataSource_p2p import DataSource

class SavedDataset(BaseModel):
    file_storage: typing.Optional[DataSource.FileOptions] = Field(default=None)
    bigquery_storage: typing.Optional[DataSource.BigQueryOptions] = Field(default=None)

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Additional context
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