fix(providers): restore legacy custom provider card

[elliottophellia]

Summary

Restore dashboard visibility for legacy custom providers stored under providers.custom.

Older Antares configurations may still use the original “Something else” provider entry. These providers remain usable by the backend, but were omitted from /api/model/options, so the frontend could not display or manage them.

This change:

• Shows configured legacy providers.custom entries in the Providers page.
• Preserves their stored label, kind, and base URL.
• Keeps the unused default custom-provider placeholder hidden.
• Allows legacy custom providers to use the existing management controls.
• Adds regression tests for both configured and unused legacy entries.

Validation

• go test ./internal/server
• go test ./...
• git diff --check