Add identifier not found error handling to evaluator

[DeveloperC286]

Summary

This PR implements proper error handling for undefined identifiers in the evaluator. Previously, accessing an undefined identifier would return Null, but now it correctly raises an "Identifier not found" error.

Key Changes

• Modified Environment.get() method to return Option<Object> instead of Object, allowing distinction between a variable that exists with a Null value and a variable that doesn't exist at all
• Added identifier expression evaluation with a new evaluate_identifier_expression() method that returns an error when an identifier is not found in the environment
• Updated expression evaluation to use the new identifier evaluation method
• Added comprehensive test suite with 10 test cases covering various scenarios:
  • Simple undefined identifiers
  • Undefined identifiers in expressions and function calls
  • Typos in function names
  • Undefined variables in function bodies
  • Undefined identifiers in conditionals and return statements
  • Scope-related cases (accessing function parameters outside their scope)

Implementation Details

• The Environment.get() method now properly propagates None through the environment chain when a variable is not found, rather than defaulting to Null
• Error messages follow the format: "Identifier not found: {identifier}."
• All test cases include lexical analysis, syntax analysis, and evaluation error snapshots
• One existing test case was updated to reflect the corrected behavior (identifier_expression_case_8 now correctly returns False instead of Null)

https://claude.ai/code/session_01Htq6Br9uauDWFGr9zPxTyc