Feature: Implement parallel test execution for mono-repos

[pradeep0153]

Closes #247.

Description

This PR executes a major architectural enhancement to support complex mono-repo setups. Repopilot currently relies on a single sandbox.run_tests() execution blocking the main agent loop.

When working with large mono-repos (e.g., a project with a Python backend and a React frontend), modifying a shared type definition or API schema might require running both pytest (backend) and vitest (frontend) to guarantee that the modification didn't break contracts on either side of the stack. Because sandbox.py only accepts a single runner, the agent was mathematically blind to failures in the secondary test suite until a human caught it in CI. We needed to implement concurrent execution across multiple runners to guarantee full-stack validation.

Changes Made

• CLI Argument Updates: Updated main.py to allow passing the --runner argument multiple times (e.g., --runner pytest --runner vitest).
• Concurrent Sandbox Refactor: Refactored SubprocessSandbox and DockerSandbox to support executing multiple commands concurrently using asyncio or concurrent.futures.
• Exit Code Aggregation: The sandbox now mathematically aggregates the exit codes.
• LLM Error Feedback: If any suite fails, the agent loop concatenates the failed output and feeds it back to the LLM in the next retry iteration, ensuring the model fixes both sides of the mono-repo simultaneously.

Impact

• Mono-repo Compatibility: Blindly running a single test suite in a multi-language monorepo is a fatal architectural limitation that guarantees broken integrations. By engineering this parallel execution pipeline, we mathematically guarantee full-stack validation. The agent can now successfully refactor cross-boundary schemas and ensure that both the frontend and backend remain perfectly synchronized, drastically expanding Repopilot's utility.

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