[Feature] Implement string.contains() per CEL spec section 6.2.5

[Quantum0uasar]

Summary

The CEL specification (section 6.2.5) defines string.contains(string) -> bool as a standard string function. Currently, calling .contains() on a StringType raises CELEvalError: no such overload in cel-python.

Steps to reproduce

import celpy

env = celpy.Environment()
ast = env.compile('"hello world".contains("world")')
prog = env.program(ast)
result = prog.evaluate({})
print(result)  # Expected: true

Actual behaviour

celpy.celenv.CELEvalError: no such overload

Expected behaviour

contains() should return true when the substring is present and false otherwise, matching the Go and Java CEL implementations.

Proposed fix

Add a contains method to StringType in src/celpy/celtypes.py:

def contains(self, other: 'StringType') -> BoolType:
    return BoolType(str(other) in str(self))

And register the overload in the function dispatch table so that "x".contains("y") is correctly dispatched.

Related

• Issue Question usage of .contains() #128 (user question about .contains() not working)
• CEL spec reference: https://github.com/google/cel-spec/blob/master/doc/langdef.md#standard-definitions