[Vardhman Gupta (Kaap10)]

Description

Fixes SearchReplaceConverter to validate each replacement template in replace against the compiled regex pattern during initialization, preventing unhandled runtime crashes (re.error: bad escape, re.error: invalid group reference, IndexError: unknown group name).

• In SearchReplaceConverter.__init__, iterates through self._replace_list and verifies each template compiles cleanly against self._compiled_regex by testing substitution on an empty string (self._compiled_regex.sub(item, "")).
• Catches (re.error, IndexError) and raises a descriptive ValueError(f"Invalid regex replacement template in replace: {e}") from e.
• Updates constructor docstring Raises section to document the validation error.
• Preserves full backward compatibility for valid capture group backreferences (\1, \g<name>) as used in 1_text_to_text_converters.ipynb and test_prompt_template_converter.py.
• Added unit tests in test_search_replace_converter.py covering invalid escape sequences (e.g. Windows file paths with \U or \u), invalid positional capture groups, unknown named capture groups, and invalid templates in replacement lists, alongside positive regression tests for valid group backreferences.

Demonstration

# Before:
converter = SearchReplaceConverter(pattern="PLACEHOLDER", replace=r"C:\Users\admin")
# Succeeded silently in __init__, but crashed at runtime in convert_async:
# re.error: bad escape \U at position 2

# After:
converter = SearchReplaceConverter(pattern="PLACEHOLDER", replace=r"C:\Users\admin")
# Raises ValueError immediately during initialization:
# ValueError: Invalid regex replacement template in replace: bad escape \U at position 2

Closes #4076

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