test: track active LiteLLM DeepSeek pricing (#3161)

## 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

- [x] 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

- [x] Unit tests pass (`pytest`)
- [x] Linting passes (`ruff check .`)
- [x] Type checking passes (`mypy headroom`)
- [ ] New tests added for new functionality
- [ ] Manual testing performed

### Test Output

```text
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

- [x] I have performed a self-review
- [x] This PR is ready for review

## Checklist

- [x] My code follows the project's style guidelines
- [x] I have performed a self-review of my code
- [x] I have commented my code where needed
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [x] Existing tests prove the fix is effective
- [x] New and existing unit tests pass locally with my changes
- [x] 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.

tests/test_providers/test_deepseek.py

@@ -95,9 +95,8 @@ class TestDeepSeekLiteLLMInjection:
         if not LITELLM_AVAILABLE:
             pytest.skip("litellm not available")
         flash = litellm.model_cost["deepseek-v4-flash"]
-        assert flash["input_cost_per_token"] == 0.14 / 1_000_000
-        assert flash["output_cost_per_token"] == 0.28 / 1_000_000
-        assert flash["cache_read_input_token_cost"] == 0.0028 / 1_000_000
+        assert flash["input_cost_per_token"] > 0
+        assert flash["output_cost_per_token"] > 0
         assert flash["litellm_provider"] == "deepseek"
 
     def test_deepseek_v4_pro_litellm_pricing(self):
@@ -106,9 +105,8 @@ class TestDeepSeekLiteLLMInjection:
         if not LITELLM_AVAILABLE:
             pytest.skip("litellm not available")
         pro = litellm.model_cost["deepseek-v4-pro"]
-        assert pro["input_cost_per_token"] == 0.435 / 1_000_000
-        assert pro["output_cost_per_token"] == 0.87 / 1_000_000
-        assert pro["cache_read_input_token_cost"] == 0.003625 / 1_000_000
+        assert pro["input_cost_per_token"] > 0
+        assert pro["output_cost_per_token"] > 0
         assert pro["litellm_provider"] == "deepseek"
 
     def test_cost_per_token_resolves_deepseek_v4_flash(self):
@@ -130,8 +128,13 @@ class TestDeepSeekLiteLLMInjection:
             prompt_tokens=1_000_000,
             completion_tokens=1_000_000,
         )
-        assert input_cost == pytest.approx(0.14, rel=0.01)
-        assert output_cost == pytest.approx(0.28, rel=0.01)
+        active_pricing = litellm.model_cost["deepseek-v4-flash"]
+        assert input_cost == pytest.approx(
+            active_pricing["input_cost_per_token"] * 1_000_000,
+        )
+        assert output_cost == pytest.approx(
+            active_pricing["output_cost_per_token"] * 1_000_000,
+        )
 
     def test_resolve_litellm_model_prefixes_deepseek(self):
         from headroom.pricing.litellm_pricing import resolve_litellm_model