Added minimum in-memory cache size

[jrhyness]

Description
If the cache size is not specified via --cache and there is no available memory reported, it gets set to 0 and qualified counters cannot be cached. This PR introduces a minimum cache size, as proposed in the linked issue.

Related Issue
#466

Tests
With no available memory reported:

{"timestamp":"2026-01-29T17:00:56.598806Z","level":"WARN","fields":{"message":"No cache size provided. Using 1000 entries (70% of 0MB available memory with a 1000 entry minimum)."},"target":"limitador_server"}

With available memory:

{"timestamp":"2026-01-29T17:03:42.177875Z","level":"WARN","fields":{"message":"No cache size provided. Using 144703856 entries (70% of 15771MB available memory with a 1000 entry minimum)."},"target":"limitador_server"}

[adam-cattermole]

I'm not sure this is entirely safe - if 1000 counters is > free_mem won't we oom?

[jrhyness]

Thanks. Good point, and would oom happen if the server was explicitly started with a --cache size ?

[adam-cattermole]

Thanks for the PR! For more context - this issue arose as on an overcommitted machine I had free_size report 0 and use a cache size of 0 (so all of my qualified counters were thrown away). My expectation was more so that if the size (70% of free mem) was below some threshold (unsure on the best value for this) we just exit out with an error.

[jrhyness]

#466

Thanks for the PR! For more context - this issue arose as on an overcommitted machine I had free_size report 0 and use a cache size of 0 (so all of my qualified counters were thrown away). My expectation was more so that if the size (70% of free mem) was below some threshold (unsure on the best value for this) we just exit out with an error.

I hit this as well. In my case, on a Mac, sys.available_memory() reported 0 but sys.free_memory() was quite a bit more. I'll revisit this. Thanks again.