Fix: normalize bytes/str handling in calculate_valid_match_lengths

[ajbmachon]

Summary

• Normalize bytes/str and ignore_patterns types before regex matching in calculate_valid_match_lengths (python/helpers/strings.py).
• Replace mutable default list with None to avoid shared state.

Rationale

Prevents TypeError in re.match when bytes and str are mixed (e.g., shell_ssh.py uses bytes patterns). Ensures robust behavior across call sites.

Test plan

• Execute SSH shell flow that trims echoed commands (python/helpers/shell_ssh.py) and verify no exceptions and correct trimming.
• Run basic unit checks if available.

Affected files

• python/helpers/strings.py