feat: add requirements and preconditions to gen slots

[jakelorocco]

Generative slot tests and examples all passed / ran.

Closes #190 and #216

Features:

• added context/backend version of the function call for generative slots
• fixed function signature when output as text for the llm; used to have additional parens like def test((one: int, two: bool, three:str) -> int:)
• fixed string param representation; now string args show as escaped:
  previous: Arguments:... - three: (type: <class 'str'>)
  now: Arguments:... - three: "" (type: <class 'str'>)
• add post conditions (requirements) to the docstring (note these only get checked if you pass in a SamplingStrategy)
• you can use sampling strategies but cannot get back a sampling result
• added preconditions
  • does a basic validation call
  • if preconditions fail, an exception is raised with the validation results as a field
• added examples / tests for these new functionalities

Semi-related changes and open questions:

• for this PR, I modified our sampling strategies to not pass a format directive to validate
  • open question: should we even allow passing a format argument to validate? This seems like a major footgun that would mess up basically every form of requirement validation and is unexpected.
  • open question: Requirement validation goes off last output; I think this is unexpected. It also means that requriement validation errors if you don't pass a ModelOutputThunk as the output parameter.

Examples of the templated output:

• precondition validation prompt:

Please check if the following arguments satisfy the precondition.
Reply with 'yes' if the precondition is satisfied and 'no' otherwise.
Do not include any other text in your response.

Arguments:
- one: 1  (type: <class 'int'>)
- two: False  (type: <class 'bool'>)
- three: "hello"  (type: <class 'str'>)

Precondition: args should all be numbers

• with postconditions:

Your task is to imitate the output of the following function for the given arguments.
Reply Nothing else but the output of the function.

Function:
def listify(one, two, three) -> list[int]:
    """Create a list from the parameters.

    Postconditions:
        - the list should be sorted
        - each item should be a number between 0 and 10
    """

Arguments:
- one: 1  (type: <class 'int'>)
- two: False  (type: <class 'bool'>)
- three: "hello"  (type: <class 'str'>)

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