fix(proxy): populate contribution USD savings

[ketpatil77]

Description

Populate session contribution USD totals for compression and cache reads.

AnthropicHandlerMixin now records both compression savings and cache-read savings in dollars instead of leaving those fields at 0.

Changes Made

• headroom/proxy/savings_tracker.py: added _estimate_cache_savings_usd() alongside the existing compression savings helper
• headroom/proxy/handlers/anthropic.py: pass computed compression and cache savings USD into update_contribution()
• tests/test_proxy_cache_savings_usd.py: regression tests for the new cache savings helper

Testing

• pytest tests/test_proxy_cache_savings_usd.py
• ruff check headroom/proxy/savings_tracker.py headroom/proxy/handlers/anthropic.py tests/test_proxy_cache_savings_usd.py

Impact

Contribution savings now show nonzero dollar values when pricing is available.

[github-actions]

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