fix: resolve lite mode backend issues and add sequential strategy default

[robinbraemer]

Fixes #564

Complete solution for console spam and strategy consistency in Gate Lite mode.

Issues Fixed ✅

1. Console Log Spam

• Root cause: Connection refused errors logged at INFO level
• Solution: Added smart detection in dialRoute to use debug level (V=1) for connection refused
• Result: No console spam when backends are down

2. Backend Cycling

• Root cause: Complex strategy logic broke simple backend retry
• Solution: Restored simple pop-first approach with proper strategy integration
• Result: All backends are tried in correct order

3. Fallback Response

• Solution: Refactored for better testability and error handling
• Result: Proper fallback when all backends fail

New Feature: Sequential Strategy ✨

Added Sequential Strategy Enum

• ✅ Added sequential strategy as explicit option
• ✅ Made sequential the default when no strategy is defined
• ✅ Updated documentation and config files
• ✅ Maintains all existing strategy behaviors

Strategy Behavior

# Default behavior (no strategy defined)
backend: [server1, server2, server3]
# → Tries in order: server1 → server2 → server3

# Explicit strategies  
strategy: sequential     # Same as default
strategy: random        # Random selection
strategy: round-robin   # Cycling rotation
strategy: least-connections  # Connection-based
strategy: lowest-latency     # Latency-based

Technical Implementation

Smart Error Handling

// Connection refused → Debug level (no spam)
if IsConnectionRefused(err) {
    v = 1  // Debug level
}

Strategy Integration

case "": 
    // Default to sequential when no strategy defined
    return sm.sequentialNextBackend(log, backends)

Comprehensive Testing

Now includes complete test coverage:

• ✅ All 5 strategies individually tested
• ✅ Default behavior verification
• ✅ Connection refused error handling
• ✅ Backend cycling and fallback scenarios
• ✅ Strategy isolation and edge cases
• ✅ Real integration tests

Breaking Changes

None - All existing configurations continue to work:

• Empty strategy → sequential (same predictable behavior)
• Explicit strategies → work exactly as before
• All config formats remain compatible

Summary

This provides the complete solution with:

• 🚫 No console spam (smart error verbosity)
• 📋 Clear strategy documentation (sequential default)
• 🔧 Proper strategy implementation (all 5 strategies tested)
• 🛡️ Comprehensive test coverage (prevents future regressions)

The fix addresses all reported issues while adding the missing sequential strategy enum and ensuring consistent, well-documented behavior.

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