🪲 BUG-#82: Fix backoff mutating retry_delay permanently across calls

[FernandoCelmer]

Description

• dotflow/core/action.py — Use local variable current_delay instead of mutating self.retry_delay during backoff
• tests/core/test_action.py — Add test_backoff_does_not_mutate_retry_delay to verify retry_delay is preserved after retries

Motivation and Context

When @action(retry=N, backoff=True) is used, the exponential backoff logic modifies self.retry_delay in place. Since the Action instance is shared across all calls, subsequent calls start with the last doubled delay instead of the original value.

Closes #82

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