fix: Keep analytics events with zero context cache capacity

[fallintoplace]

What

• Keep analytics events working when context_keys_capacity=0.

Why

• The empty cache raises KeyError and drops identify, custom, and feature events.

Implementation

• Skip cache writes when capacity is zero or negative.

---

Note

Overview
Fixes analytics delivery when context_keys_capacity=0 by treating zero or negative LRU capacity as a no-op cache.

SimpleLRUCache.put now returns False immediately when capacity <= 0, so nothing is stored and the event pipeline no longer hits errors from an effectively empty context-key cache (which previously dropped identify, custom, and feature events).

With capacity zero, context deduplication is disabled: each event is treated as a new context, so payloads include repeated index/identify entries (covered by new sync and async event-processor tests, plus LRU unit tests for capacities 0 and -1).

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[jsonbailey]

@fallintoplace thank you for submitting this PR. I am going to review this with our team as this particular issue is impacts more than just the python server sdk and we need to decide how we want to address this for all SDKs. Can you share more about your use case for setting it to 0 or -1 so I can bring that up in the discussion.