Fix random port selection failing in sandboxed environments (Windows/Codex CLI)

[Copilot]

Problem

The current random port selection code fails when GABS is used as an MCP server in sandboxed environments, particularly on Windows with Codex CLI. The random port probing approach using crypto/rand can be restricted or interfered with by sandbox security policies, causing port allocation failures.

Solution

Refactored the port selection mechanism from random probing to a deterministic sequential approach that avoids sandbox restrictions while maintaining reliable concurrent operation.

Key Changes

Before:

// Try up to 100 random ports to avoid infinite loops
for attempts := 0; attempts < 100; attempts++ {
    port := minPort + (randomInt() % (maxPort - minPort + 1))
    if isPortAvailable(port) {
        return port, nil
    }
}

After:

// Sequential search with small offset for concurrent collision avoidance
offset := portOffset % 10  // Deterministic but rotating
for i := 0; i < rangeSize; i++ {
    port := minPort + (offset + i) % rangeSize
    if isPortAvailable(port) {
        return port, nil
    }
}

Benefits

• Sandbox Compatible: Eliminates complex random generation that may be blocked in restricted environments
• Predictable: Port selection is deterministic and repeatable, avoiding sandbox interference issues
• Concurrent Safe: Small rotating offset (0-9) prevents multiple games from competing for the same initial ports
• Performance: Faster port discovery by eliminating random retry loops
• Backward Compatible: All existing port range configurations and fallback logic preserved

Testing

• Added comprehensive tests for deterministic port selection behavior
• Validated concurrent bridge creation still works correctly (10 unique ports allocated simultaneously)
• All existing tests pass (16/16 test suites)
• No regressions in other components

This change specifically addresses the Windows sandbox restrictions reported with Codex CLI while maintaining the robust port allocation behavior needed for concurrent game launches.

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