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.

Closes #1751

Type of Change

• Bug fix (non-breaking change that fixes an issue)

Changes Made

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

Testing

• Unit tests pass (pytest)
• Linting passes (ruff check .)
• Type checking passes (mypy headroom)
• New tests added for new functionality
• Manual testing performed

Test Output

$ pytest tests/test_proxy_cache_savings_usd.py
Focused cache-savings regression suite passed during local verification before PR submission.

$ ruff check headroom/proxy/savings_tracker.py headroom/proxy/handlers/anthropic.py tests/test_proxy_cache_savings_usd.py
Lint checks passed during local verification before PR submission.

Real Behavior Proof

• Environment: Local Headroom proxy savings-tracker path for Anthropic contribution updates.
• Exact command / steps: Verified helper and handler wiring that computes compression and cache-read USD totals before calling update_contribution().
• Observed result: Contribution savings fields populate with nonzero dollar estimates when model pricing is available.
• Not tested: Full live Anthropic upstream session in this verification pass.

Review Readiness

• I have performed a self-review
• This PR is ready for human review

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have updated the CHANGELOG.md if applicable