This PR addresses the refactoring needs identified in #[issue_number] by transforming Junction from a monolithic service into a clean, modular architecture with comprehensive testing.

Problem

The original service suffered from several maintainability issues:

1. Poor readability: The core fallback logic was contained in a single 187-line handleRpcRequest function that was difficult to understand and maintain
2. Basic routing: Used raw Deno.serve with manual request processing instead of a proper framework
3. No tests: Zero test coverage meant no confidence in refactoring or adding features

Solution

🔧 Modular Architecture Refactor

Broke the monolithic RPC handler into focused, single-responsibility modules:

• src/auth.ts - Authentication and bypass token validation
• src/routing.ts - URL pattern matching and network validation
• src/rpc_client.ts - RPC forwarding with timeout and fallback logic
• src/rate_limiter.ts - IP-based rate limiting (unchanged)

The main handleRpcRequest function was reduced from 187 to 89 lines and now serves as a clean orchestrator.

🚀 Oak Framework Integration

Replaced raw Deno.serve with a professional Oak-based server:

• Middleware stack: CORS, logging, and rate limiting as proper middleware
• Clean routing: URL patterns handled by Oak router with parameter extraction
• Request processing: Leverages Oak's context system for cleaner request/response handling

Both server implementations are maintained for backward compatibility via USE_OAK_SERVER flag.

✅ Comprehensive Test Coverage

Added a complete test suite covering all required scenarios:

// All required test cases implemented:
- First RPC successful (normal case)
- First RPC fails, second successful (returns 2nd response)  
- First RPC hangs, second successful (returns 2nd response)
- All RPCs fail (returns status and body of last request)

Plus comprehensive unit tests for individual modules and integration tests for complete request flows.

Impact

• Maintainability: Code is now modular with clear separation of concerns
• Testability: 100% coverage of fallback scenarios plus edge cases
• Developer Experience: Oak framework provides familiar middleware patterns
• Reliability: Comprehensive tests ensure behavior is preserved during future changes
• Documentation: Updated README with architecture overview and development guides

The refactor maintains identical runtime behavior while dramatically improving code quality and maintainability.

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