Add Mock HTTP Client for Integration Tests

Closes #440

This PR introduces a mock HTTP client system to enable integration tests without making real HTTP requests to external services like LLM providers.

Features

• Dual-mode operation: Real mode (actual requests) or Mock mode (cached responses)

• Transparent API: Implements the same interface as reqwest::Client

• Automatic response caching in Real mode

• Configurable cache directory

• Support for updating cached responses

• Offline mode support for skipping tests without available mocks

Usage

Running Integration Tests

# Real mode (requires API keys)
cargo test
Mock mode (uses cached responses)
USE_MOCK_PROVIDER=true cargo test
Offline mode (skips tests without mocks)
USE_MOCK_PROVIDER=true OFFLINE_MODE=true cargo test

Recording Mock Data

# For integration tests
export OPENROUTER_API_KEY=your_api_key
cargo run --bin record_integration_test_mocks
For general usage
cargo run –bin record_mocks
cargo run –bin update_mocks

Environment Variables

Mock File Format

• Stored in tests/fixtures/llm_mocks/

• JSON files named using a hash of the request

• Each file includes method, URL, headers, request body, response status, headers, and body

Helper Utilities

• get_test_client(): Returns a client based on current environment

• is_offline_mode(): Detects if offline mode is active

• skip_if_offline_without_mock(): Skips test if mock is missing in offline mode

Benefits

• Faster test runs

• No API rate limits or external dependencies

• Reduced testing cost

• Supports offline development and CI environments

• Deterministic test behavior

/claim #440