🪲 BUG-#69: Retry and surface clear error on empty response

[FernandoCelmer]

Description

• pycodeloop/core/agent.py: Added _MAX_EMPTY_RESPONSE_RETRIES = 2 module-level constant and a retry loop inside run() that detects empty responses (no text, no tool calls) and retries up to 2 times before returning a descriptive error string naming the model.
• tests/core/test_agent.py: Added two regression tests — one verifying recovery after transient empty responses, one verifying the give-up error message after exhausting retries.

Motivation and Context

Weak models (e.g. Ollama qwen2.5:3b in agent mode) can return a fully empty message with no text and no tool calls. The agent loop previously treated this as a normal "done" turn and silently returned "". This change retries up to 2 times and, if all attempts fail, surfaces a clear error message naming the model instead of silently succeeding with empty output.

Closes #69

Types of changes

• Bug fix (change that fixes an issue)
• New feature (change which adds functionality)
• Documentation

Checklist

• I have performed a self-review of my own code
• I have added tests that prove my fix is effective or that my feature works
• I have updated the CHANGELOG
• I have updated the documentation accordingly

[FernandoCelmer]

🔍 Code Review

Code issues found: 2

#	Severity	Comment
1	[Blocking]	Usage silently lost on give-up path
2	[Suggestion]	Session left inconsistent on early return