[feat]: sort /v1/models response alphabetically by model ID

[levi-zheng]

Feature Request

Summary

The /v1/models endpoint currently returns models in provider declaration order
(the order they appear in config.yaml). This makes the model list hard to
navigate in downstream tools like Cursor, Cline, and Windsurf, especially when
multiple providers are configured.

Expected Behavior

Models in the /v1/models response should be sorted alphabetically by id,
so the list is predictable and easy to scan regardless of provider declaration order.

Current Behavior

Models are returned in the order they are registered across providers. When
multiple providers expose the same alias (e.g. gemini-3.1-pro from both
antigravity and anyrouter), the merged list order is effectively arbitrary.

Motivation

• Most OpenAI-compatible API gateways (OpenRouter, New API, One API) return
  models sorted alphabetically — this is the de facto standard
• Alphabetical order is stable and predictable; declaration order changes
  whenever providers are added or reordered in config
• The fix is low-risk: a single sort.Slice on the final model list before
  serialization, with zero impact on routing or alias resolution logic

Suggested Implementation

Sort the model list by id before returning in the /v1/models handler:

sort.Slice(models, func(i, j int) bool {
    return models[i].ID < models[j].ID
})

Optionally, this could be made configurable (e.g. model-list-sort: alphabetical)
if the maintainers prefer to keep the current behavior as default.

Environment

• CLIProxyAPI version: latest
• Multiple providers configured with overlapping model aliases