Standardize response schemas for variables and xcoms in Execution API

[rjgoyln]

Description

Currently, the Execution API has inconsistent response formats for success messages. While endpoints like connections.py return well-defined Pydantic schemas (e.g., ConnectionResponse), others like variables.py and xcoms.py return raw Python dictionaries (e.g., {"message": "..."}).

Returning raw dictionaries causes the OpenAPI specification to default to Any or object, which makes it difficult to generate strongly-typed SDKs and Clients.

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