Fix string literal corruption in Python evaluation mode

[hardbyte]

Replaces brittle string-based integer promotion with robust AST-based approach. Previously, string literals like "epa1" were incorrectly modified to "epa1.0" when floats existed in the evaluation context, causing string comparisons to fail.

The new implementation:

• Parses expressions using CEL's AST to identify actual integer literals
• Preserves string literals exactly as written
• Maintains proper array indexing (keeps indices as integers)
• Gracefully handles complex constructs like comprehensions via fallback
• Ensures consistent behavior across Python and Strict evaluation modes

Fixes string comparison expressions like var == "epa1" returning False when floats are present in context. Includes comprehensive test coverage for both evaluation modes.

Resolves #16

[Copilot]

Pull Request Overview

This PR fixes a critical bug where string literals containing numbers were incorrectly corrupted during evaluation when floats existed in the context. The solution replaces brittle string-based preprocessing with a robust AST-based approach that preserves string literals exactly while still enabling proper integer-to-float promotion for arithmetic operations.

• Implements AST-based integer promotion that distinguishes between string literals and actual integer literals
• Preserves array indexing by keeping indices as integers rather than promoting them to floats
• Adds comprehensive test coverage for the fix across both Python and Strict evaluation modes

Reviewed Changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/lib.rs	Core implementation replacing string-based integer promotion with AST-based approach
tests/test_issue16_string_literal_regression.py	Focused regression tests for the string literal corruption issue
tests/test_dual_mode_comprehensive.py	Comprehensive test suite validating both evaluation modes work correctly
tests/test_enhanced_error_handling.py	Minor test update to use explicit float literal
CHANGELOG.md	Documentation of the bug fix and new AST-based approach

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