Fix: enforce positive memory_max_entries for in-memory cache

[mshr-h]

Passing invalid memory_max_entries to Cache can crash cachetool with unclear errors. This PR validates it upfront and fails with clearer messages.

Repros:

from dataclasses import dataclass
from dspy.clients.cache import Cache

@dataclass
class DummyResponse:
    message: str
    usage: dict

if __name__ == "__main__":
    memory_cache = Cache(
        enable_disk_cache=False,
        enable_memory_cache=True,
        disk_cache_dir="",
        disk_size_limit_bytes=0,
        memory_max_entries=1, # negative entry size
    )

    request = {"prompt": "Hello", "model": "openai/gpt-4o-mini", "temperature": 0.7}
    value = DummyResponse(message="This is a test response", usage={"prompt_tokens": 10, "completion_tokens": 20})
    memory_cache.put(request, value)  # -> ValueError: value too large

from dataclasses import dataclass
from dspy.clients.cache import Cache

@dataclass
class DummyResponse:
    message: str
    usage: dict

if __name__ == "__main__":
    memory_cache = Cache(
        enable_disk_cache=False,
        enable_memory_cache=True,
        disk_cache_dir="",
        disk_size_limit_bytes=0,
        memory_max_entries=None, # None
    )

    request = {"prompt": "Hello", "model": "openai/gpt-4o-mini", "temperature": 0.7}
    value = DummyResponse(message="This is a test response", usage={"prompt_tokens": 10, "completion_tokens": 20})
    memory_cache.put(request, value)  # -> TypeError: '>' not supported between instances of 'int' and 'NoneType'

[chenmoneygithub]

Thanks for the PR! Two minor comments, then we should be good to go!

[chenmoneygithub]

Thanks for addressing the comments! LGTM with 2 minor comments.

[mshr-h]

fyi: math.inf is float type so it should also accept it.

[chenmoneygithub]

yes this definitely reads weird in type hint, but that's sticking with cachetools' best practice, so acceptable.

[mshr-h]

Ok. reverted. @chenmoneygithub

[mshr-h]

nit: Maybe just float type is better than int | float or int?
https://github.com/python/typeshed/blob/main/stubs/cachetools/cachetools/__init__.pyi#L20

    def __init__(self, maxsize: float, getsizeof: None = None) -> None: ...

[mshr-h]

@chenmoneygithub
Thank you for your review! I updated the code accordingly.