AI Gateway: support multiple API keys per provider in the UI #1557

Description

The AI Gateway key failover feature supports up to 5 centralized API keys per provider (OpenAI and Anthropic), but the dashboard provider form only allows entering a single key. Multiple keys can currently only be configured via the API with CreateAIProviderRequest and UpdateAIProviderRequest.

This issue covers:

• Update the provider form to support adding, viewing, and removing multiple API keys.
• Only enable multi-key input for OpenAI and Anthropic provider types.
• Show a validation error when more than 5 keys are configured.
• Update docs/ai-coder/ai-gateway/setup.md (Dashboard section) to reflect the new multi-key UI.
• Update docs/ai-coder/ai-gateway/providers.md to mention dashboard support alongside the API.