Add cost methods for EVCache, Kafka, Cassandra with CostAwareModel mixin

[homatthew]

Summary

• Introduce CostAwareModel mixin for models that implement cost calculation methods
• Add cluster_costs() and service_costs() to EVCache, Kafka, and Cassandra models
• Mixin pattern avoids pylint abstract-method warnings while enforcing implementation

Design

Models that compute costs inherit from both CapacityModel and CostAwareModel:

class NflxEVCacheCapacityModel(CapacityModel, CostAwareModel):
    @staticmethod
    def cluster_costs(...) -> Dict[str, float]:
        ...

This mixin can be folded into CapacityModel once all models implement cost methods.

Test plan

• All 224 Netflix tests pass
• Cost regression tests pass
• Pylint passes without warnings

🤖 Generated with Claude Code

[homatthew]

This can be folded into the original cap planning models once we have it implemented across the board. aurora / rds are a little tricky for example

[homatthew]

Oops. This is a weird GPT comment. I will fix this

[homatthew]

I did not mean to merge this 🫤 . I'll recreate it in stacked commit style

[homatthew]

Continued in #212