Fix OpenAI API errors, session logging warnings, and infinite loops in AgentAlpha

[Copilot]

This PR fixes three critical bugs identified in the AgentAlpha GitHub PR review workflow that were causing execution failures:

🔧 Issues Fixed

1. OpenAI API "Unknown parameter: 'tool_choice.function'" Error

Problem: PlanningService was using an outdated OpenAI API format for the tool_choice parameter, causing requests to fail with:

System.InvalidOperationException: OpenAI API error: BadRequest – {
  "error": {
    "message": "Unknown parameter: 'tool_choice.function'.",
    "type": "invalid_request_error",
    "param": "tool_choice.function",
    "code": "unknown_parameter"
  }
}

Solution: Updated the tool_choice format to use the correct API specification:

// Before (causing errors)
ToolChoice = new { type = "function", function = new { name = "create_execution_plan" } }

// After (working correctly)
ToolChoice = "required"

2. Session Activity Logger "Cannot log activity - no current session set" Warnings

Problem: When using the simple ExecuteAsync(string task) method, no session was created, causing repeated warnings:

warn: AgentAlpha.Services.SessionActivityLogger[0]
      Cannot log activity - no current session set

Solution: Added automatic default session creation when no session is provided, ensuring activity logging works in all execution scenarios.

3. Infinite Loop When Tools Return Errors

Problem: When GitHub API returned 404 errors (or similar), the agent would get stuck repeating the same response indefinitely:

AI: It seems there was an issue accessing the pull request details...
AI: It seems there was an issue accessing the pull request details...
AI: It seems there was an issue accessing the pull request details...

Solution: Implemented comprehensive repetitive response detection:

• Tracks the last 5 assistant responses
• Detects when >80% word similarity indicates repetition
• Breaks conversation loops after 3 iterations if stuck
• Provides guidance messages to help the agent move forward
• Logs warnings when repetitive behavior is detected

🧪 Testing

• ✅ All existing tests pass (207 tests, no regressions)
• ✅ Build succeeds without warnings
• ✅ Manual testing confirms all three issues are resolved

📁 Files Modified

• src/Agent/AgentAlpha/Services/PlanningService.cs - Fixed OpenAI API compatibility
• src/Agent/AgentAlpha/Services/TaskExecutor.cs - Added session initialization and loop detection
• src/Agent/AgentAlpha/Services/ConversationManager.cs - Added repetitive response detection
• src/Agent/AgentAlpha/Interfaces/IConversationManager.cs - Added repetition detection interface

The AgentAlpha GitHub PR review workflow should now execute successfully without the reported errors.

Fixes #65.

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