fix(generate_script): fallback to auto mode on invalid custom_script

[fancyboi999]

Summary

• fix invalid custom_script handling in generate_script
• when custom_script format validation fails, fallback to normal auto-generation path instead of returning invalid payload
• initialize and return script_title safely from parsed JSON to keep return path deterministic

Why

Previously, invalid custom_script input could be returned directly, which propagates malformed structure to downstream nodes. This creates avoidable runtime instability.

Changes

• src/open_storyline/nodes/core_nodes/generate_script.py
  • valid custom_script: unchanged behavior (immediate return)
  • invalid custom_script: log and fallback to auto generation
  • return title through explicit script_title default

Validation

• python3 -m py_compile src/open_storyline/nodes/core_nodes/generate_script.py
• regression script coverage:
  • invalid custom_script -> fallback auto generation
  • valid custom_script -> no llm call
  • invalid llm json -> raises expected ValueError (no variable undefined issue)

Scope

• code only, no docs included