Add OpenRouter provider support for unified LLM API access

[Copilot]

This PR adds OpenRouter as a first-class LLM provider, enabling users to route requests through OpenRouter's unified API to access multiple upstream models (OpenAI, Anthropic, Google, open models, etc.) with a single API key.

What's Added

New OpenRouter Provider: Implements OpenRouterProvider extending BaseLLMProvider using native fetch for explicit control and lighter footprint. Features include:

• Default model: openai/gpt-4o-mini (cost-effective choice)
• OpenAI-compatible API format with proper error handling
• Optional branding headers (HTTP-Referer, X-Title) via environment variables
• Comprehensive error normalization for HTTP errors, malformed responses, and network issues

Configuration Support:

• Added openrouter to provider union type in schema
• Environment variable support: CONTEXT_OPT_LLM_PROVIDER=openrouter and CONTEXT_OPT_OPENROUTER_KEY
• Configuration validation ensuring API key is required when provider is selected
• Sanitized config output includes hasOpenrouterKey boolean

Factory Integration: Updated LLMProviderFactory to create OpenRouter provider instances seamlessly alongside existing providers (gemini, claude, openai).

Comprehensive Testing: Added 11 unit tests covering:

• Provider properties and instantiation
• API key validation
• Successful requests with proper headers and body format
• HTTP error handling with and without error JSON
• Malformed response handling (missing/empty choices)
• Network error handling
• Optional branding header inclusion

Usage

# Required
CONTEXT_OPT_LLM_PROVIDER=openrouter
CONTEXT_OPT_OPENROUTER_KEY=<your-openrouter-api-key>
CONTEXT_OPT_ALLOWED_PATHS=<your-allowed-paths>

# Optional branding headers
CONTEXT_OPT_APP_URL=<your-site-url>
CONTEXT_OPT_APP_NAME=<your-app-name>

Technical Details

• API Endpoint: https://openrouter.ai/api/v1/chat/completions
• Request Format: OpenAI-compatible with model, messages, temperature, max_tokens
• Error Handling: Structured error responses matching existing provider patterns
• Dependencies: No additional dependencies required (uses native fetch)

All existing functionality remains unchanged. The implementation follows the same patterns as other providers and maintains backward compatibility.

Fixes #1.

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Pull Request Overview

This PR adds OpenRouter as a new LLM provider to enable unified API access to multiple upstream models through a single API key. OpenRouter acts as a router that provides access to OpenAI, Anthropic, Google, and other models via an OpenAI-compatible API format.

• Implements OpenRouterProvider class with native fetch for lightweight HTTP requests
• Adds OpenRouter configuration support with environment variables and validation
• Integrates OpenRouter into the existing provider factory system

Reviewed Changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/providers/openrouter.ts	New OpenRouter provider implementation with error handling and optional branding headers
src/providers/factory.ts	Added OpenRouter case to provider factory for instantiation
src/config/schema.ts	Extended provider union type and config interface to include OpenRouter
src/config/manager.ts	Added OpenRouter environment variable support and validation logic
test/openrouter.test.ts	Comprehensive test suite covering all provider functionality and error cases