_build_test_path fallback branch never executes (dead code in critic_agent.py)

[VinaySampath14]

In src/agents/critic_agent.py, the _build_test_path function has dead
code in its else branch:

if os.path.exists(full_path):
    node_ids.append(test_id)
    seen_files.add(file_part)
else:
    # File not found at this commit — fall back to file name only
    if file_part not in seen_files and os.path.exists(full_path):
        seen_files.add(file_part)

The inner os.path.exists(full_path) check in the else branch will always
return False — we only reach the else when os.path.exists(full_path) is
already False. The fallback never runs.

Fix: remove the redundant os.path.exists(full_path) check from the else
branch so the fallback actually executes.