feat(combos): add custom context length controls

[mahdiwafy]

Summary

• Adds custom advertised context_length configuration for model combos.
• Adds preset context length buttons and model capacity warning notes.
• Adds the same context length editing flow to media provider combo pages.
• Invalidates model-list cache after combo create/update/delete.

Scope

Focused only on combo context length configuration and related cache invalidation.

Verification

• npm run lint:undef