feat: add env var support for custom endpoint default models

[carbap]

Summary

Adds support for configuring the default models of custom endpoints via environment variables.

The models.default property can include ${ENV_VAR} references in addition to literal model names. If an env var resolves to a comma-separated string, it is automatically expanded into multiple model names.

Discussion: #12664
Documentation PR: LibreChat-AI/librechat.ai#592

Change Type

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

Testing

Here is an example configuration:

librechat.yaml

endpoints:
 custom:
   - name: "Mistral"
     apiKey: "${MISTRAL_API_KEY}"
     baseURL: "${MISTRAL_BASE_URL}"
     models:
       default: ["${MISTRAL_MODEL}", "${MISTRAL_OTHER_MODELS}"]
       fetch: false
     modelDisplayLabel: "Mistral"
     dropParams: ["stop", "user", "frequency_penalty", "presence_penalty"]

.env

MISTRAL_MODEL=mistral-small-latest
MISTRAL_OTHER_MODELS=mistral-medium-latest,mistral-large-latest

Checklist

Please delete any irrelevant options.

• I have made pertinent documentation changes
• I have written tests demonstrating that my changes are effective or that my feature works
• Local unit tests pass with my changes
• A pull request for updating the documentation has been submitted.