fix: ensure retry=0 executes the task exactly once

[avarga1]

Description

• dotflow/core/action.py: Use max(1, self.retry) to ensure retry=0 executes the task at least once
• tests/core/test_action.py: Add test verifying retry=0 runs the task exactly once

Motivation and Context

With retry=0, range(1, 0+1) produces an empty range, so the task never executes and returns None silently. The fix guarantees at least one execution attempt.

Closes #143

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

[avarga1]

Good catch — you're right, the raise guard was still comparing against self.retry (always False when retry=0 since attempt starts at 1). Fixed by extracting max_attempts = max(1, self.retry) and using it in both the loop bound and the raise guard. Also added test_retry_zero_raises_on_failure as suggested.