test: track active LiteLLM DeepSeek pricing

[JerrettDavis]

Description

Keep the LiteLLM DeepSeek V4 integration tests compatible with upstream-owned pricing entries. LiteLLM now publishes these models directly, so Headroom correctly preserves upstream values instead of installing its fallback values; the tests must validate the active entry rather than require fallback prices.

Related: #3157

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Performance improvement
• Code refactoring (no functional changes)

Changes Made

• Validate that active upstream DeepSeek V4 price entries contain positive input and output prices.
• Compare cost_per_token results with the active LiteLLM model-cost entry.
• Preserve the existing fallback-price and non-overwrite coverage.

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

python -m pytest tests/test_providers/test_deepseek.py -q
20 passed in 4.63s

ruff check tests/test_providers/test_deepseek.py
All checks passed!

ruff format --check tests/test_providers/test_deepseek.py
1 file already formatted

pre-commit: Ruff alignment, merge-conflict check, Ruff, Ruff format, and mypy all passed

Real Behavior Proof

• Environment: Windows, Python 3.13.13, LiteLLM model-cost data available.
• Exact command / steps: python -m pytest tests/test_providers/test_deepseek.py -q
• Observed result: all 20 DeepSeek provider and pricing tests pass against the active LiteLLM entries.
• Not tested: provider API calls; this change only concerns local pricing metadata assertions.

Runtime Rollout Safety

• Rollout-managed feature(s): None.
• Minimum rollout channel: N/A.
• Stable/default behavior changed: No runtime behavior changes.
• Kill switch / disable path: N/A.
• Unsafe override required: No.
• Qualification impact: Restores deterministic CI coverage for upstream-owned pricing entries.
• Rollback path: Revert this test-only commit.

Review Readiness

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

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have commented my code where needed
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Existing tests prove the fix is effective
• New and existing unit tests pass locally with my changes
• I did not edit CHANGELOG.md — it is generated by release-please from my Conventional Commit PR title (a CI guard enforces this)

Screenshots (if applicable)

N/A — test-only change.

Additional Notes

Documentation changes are not applicable because runtime behavior and public APIs are unchanged.

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Codecov Report

✅ All modified and coverable lines are covered by tests.

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