## Summary
This PR adds user-friendly error handling for AI chat features,
specifically for **billing credits exhausted** and **API key not
configured** errors.
## Changes
### Backend
- Added `BILLING_CREDITS_EXHAUSTED` exception code with 402 status
- Added `API_KEY_NOT_CONFIGURED` exception code with 503 status
- Added billing check before AI chat streaming in
`agent-chat.controller.ts`
- Added error code to HTTP exception response body for frontend error
type detection
- Created `AgentRestApiExceptionFilter` for agent-specific errors
### Frontend
- Created `AIChatBanner` - reusable banner component for error/warning
messages
- Created `AIChatCreditsExhaustedMessage` - shows upgrade prompts based
on user permissions
- Created `AIChatApiKeyNotConfiguredMessage` - shows configuration
guidance with docs link
- Created `AIChatErrorRenderer` - encapsulates error type switching
logic (fixes nested ternary)
- Created `AIChatStandaloneError` - displays errors when there are no
messages
- Split `aiChatErrorUtils.ts` into separate files (1 export per file):
- `AIChatErrorCode.ts`
- `extractErrorCode.ts`
- `isAIChatErrorOfType.ts`
- `isBillingCreditsExhaustedError.ts`
- `isApiKeyNotConfiguredError.ts`
- Added comprehensive test coverage (27 tests)
### Other
- Updated trial period banner messaging
## Testing
- All lint checks pass
- All 27 new tests pass
- TypeScript typecheck passes
