🖼️ refactor: Enhance Env Extraction & Agent Image Handling

[danny-avila]

Summary

I refactored the environment variable extraction logic and unit tests while enhancing agent-specific image handling for generated images in both Stable DiffusionAPI and DALL-E integrations.

• Refactored StdioOptionsSchema to transform env variables using the new extractEnvVariable function from a dedicated utils module.
• Moved environment extraction logic from parsers to utils to avoid circular dependencies and improve clarity.
• Added comprehensive unit tests for env variable processing in mcp and data-provider, and removed deprecated tests.
• Updated the backend review workflow to run unit tests for the librechat-mcp package.
• Enhanced StableDiffusionAPI by adding agent-specific return values and a display message for generated images.
• Improved DALL-E integration by incorporating proxy support for image fetching, adjusting logging placement, and updating agent image output format using nanoid.

Change Type:

• New feature (non-breaking change which adds functionality)

Checklist:

• My code adheres to this project's style guidelines
• I have performed a self-review of my own code
• I have commented in any complex areas of my code
• My changes do not introduce new warnings
• I have written tests demonstrating that my changes are effective or that my feature works
• Local unit tests pass with my changes
• Any changes dependent on mine have been merged and published in downstream modules