feat(frontend): add AI Gateway integration for dynamic provider and model selection

Add comprehensive AI Gateway integration to Console UI with the following features:

- **Gateway Selection**: Fetch and display available AI Gateways from AI Gateway API
  - Auto-select first available gateway when creating agents
  - Show gateway ID in agent configuration details

- **Dynamic Provider Loading**: Fetch LLM providers from AI Gateway ListModelProviders API
  - Replace hardcoded provider list with dynamic data
  - Display provider logos from API response

- **Dynamic Model Loading**: Fetch models from AI Gateway ListModels API
  - Filter models by selected provider
  - Auto-select first model when provider changes
  - Show model metadata (context window, description)

- **Dual-Mode Support**:
  - AI Gateway mode: Use gateway for auth, dynamic providers/models, hide API key field
  - Legacy mode: Use API key, hardcoded providers/models (backward compatible)

- **Feature Flag**: enable-api-key-configuration-agent
  - false (default): AI Gateway mode with dynamic data
  - true: Legacy mode with hardcoded providers and API key requirement

- **API Integration**:
  - Add buf.build/redpandadata/ai-gateway dependency
  - Generate TypeScript types for Gateway, ModelProviders, and Models services
  - Create React Query hooks with proper caching and authentication
  - Configure dev server proxy for AI Gateway API (/.redpanda/api/redpanda.api.aigateway.v1)

- **Validation**:
  - API key optional when gateway is configured (proto updated with ignore = IGNORE_IF_ZERO_VALUE)
  - Gateway required when available
  - Conditional validation based on mode

- **UX Improvements**:
  - No layout shift: gateway field always visible (disabled when unavailable)
  - Loading states for gateway/provider/model dropdowns
  - No validation errors during loading transitions
  - Icons displayed for providers in both selected state and dropdown

Technical details:
- Proto generation includes AI Gateway v1 API from buf.build
- Proxy configuration in both Console UI and Cloud UI to handle CORS
- Custom transport for AI Gateway with bearer token authentication
- React Query caching with staleTime to prevent excessive refetching

Co-Authored-By: Claude Sonnet 4.5 (1M context) <noreply@anthropic.com>

Author: alenkacz