Backtick 'value' keyword in from_json struct type definitions

[tkaunlaky-e6]

Summary

• In from_json(), the schema string 'struct<value: string>' needs value backticked as `value` since it's a reserved keyword in the E6 engine's struct type parser
• Handles it at the AST level in the e6 generator's anonymous_sql — when the function is from_json, checks each Literal argument for value: and replaces with `value`:
• Does not affect valueKey, valueOf, etc. since replace("value:", ...) only matches the exact substring

Test plan

• All 47 e6 tests pass (856 subtests)
• struct<value: string, valueKey: string> → struct<value: string, valueKey: string>

[gauravdawar-e6]

Looks good!