Add substring() function to stdlib and improve logging

[hardbyte]

Add substring() function to stdlib and improve logging

This PR introduces a new cel.stdlib module with Python implementations of CEL functions missing from upstream cel-rust, and improves logging levels for execution errors.

🎯 Changes

1. New Standard Library Module

• Added python/cel/stdlib.py - Python implementations of missing CEL standard library functions
  • Implements substring(str, start, end?) function
  • Provides add_stdlib_to_context() convenience function
  • Extensible design with STDLIB_FUNCTIONS dict for future additions
  • References upstream issue: Strings are not index accessible cel-rust/cel-rust#200

2. CLI Auto-loads stdlib

• Updated python/cel/cli.py
  • CLI automatically includes all stdlib functions (no manual setup required)
  • Added substring to syntax highlighting
  • Added substring to autocomplete
  • Improved UX - stdlib functions work out of the box

3. Logging Improvements

• Updated src/lib.rs (version 0.5.3)
  • Changed execution error logging from warn! to debug! level
  • Reduces noise in production environments
  • Errors still properly propagated to Python as exceptions

4. Documentation Updates

• Updated docs/reference/cel-compliance.md

  • Added "Using cel.stdlib" section with examples
  • Clarified string indexing is not supported
  • Added function vs method syntax notes

• Updated docs/reference/cli-reference.md

  • Added "Standard Library Functions" section
  • Documented CLI auto-includes stdlib
  • Provided usage examples

• Updated error handling docs - Fixed error message assertions to use "Failed to parse expression"

🧪 Test Coverage

• Added tests/test_stdlib.py (14 tests)

  • Basic functionality tests
  • Edge case coverage
  • Integration with CEL expressions
  • Context variable interactions
  • Upstream detection tests - Will alert when cel-rust adds native support

• Updated tests/test_cli.py (2 new tests)

  • Verifies stdlib auto-loading in CLI
  • Tests stdlib functions with context variables

• All tests passing: 342 passed, 1 skipped, 7 xfailed

📝 Usage Example

Programmatic API:

import cel
from cel.stdlib import add_stdlib_to_context

context = cel.Context()
add_stdlib_to_context(context)

result = cel.evaluate('substring("hello world", 0, 5)', context)  # → "hello"

CLI (auto-included):

$ cel 'substring("hello world", 0, 5)'
hello

$ cel 'substring("hello world", 6)'
world

🔍 Design Decisions

1. Function syntax, not method syntax: substring("text", 0, 5) instead of "text".substring(0, 5)

   • Matches how Python functions are added to CEL context
   • Consistent with other custom functions pattern

2. CLI auto-loads stdlib: Better UX - functions work without setup

3. Upstream detection: Tests will fail when cel-rust adds native support, alerting us to switch to native implementation

4. Extensible pattern: Easy to add more stdlib functions in the future