[v3] Migrate nodes_string to v3 schema

[christian-byrne]

Converts all string nodes from v1 dictionary-based format to v3 schema-based format

Converted Nodes

• StringConcatenate - Concatenates two strings with optional delimiter
• StringSubstring - Extracts substring using Python slice notation
• StringLength - Returns string length
• CaseConverter - Converts text case (upper, lower, capitalize, title)
• StringTrim - Removes whitespace from strings
• StringReplace - Simple find and replace
• StringContains - Checks substring presence
• StringCompare - String comparison with multiple modes
• RegexMatch - Tests regex pattern matching
• RegexExtract - Extracts text using regex patterns and groups
• RegexReplace - Find and replace with regex support