feat(pricing): add deepseek-v4-flash to official-docs pricing snapshot

DeepSeek's /models endpoint returns no pricing, so direct-provider routes fall back to the _OFFICIAL_DOCS_PRICING snapshot. The table included deepseek-v4-pro but not the newer deepseek-v4-flash, so flash sessions reported $0.00 with cost_source "none". Add the flash entry (values from DeepSeek's official pricing page, mirroring the v4-pro entry; DeepSeek bills no separate cache-write cost) plus two regression tests.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

agent/usage_pricing.py

@@ -477,6 +477,17 @@ _OFFICIAL_DOCS_PRICING: Dict[tuple[str, str], PricingEntry] = {
         source_url="https://api-docs.deepseek.com/quick_start/pricing",
         pricing_version="deepseek-pricing-2026-05-12",
     ),
+    (
+        "deepseek",
+        "deepseek-v4-flash",
+    ): PricingEntry(
+        input_cost_per_million=Decimal("0.14"),
+        output_cost_per_million=Decimal("0.28"),
+        cache_read_cost_per_million=Decimal("0.0028"),
+        source="official_docs_snapshot",
+        source_url="https://api-docs.deepseek.com/quick_start/pricing",
+        pricing_version="deepseek-pricing-2026-05-12",
+    ),
     # Google Gemini
     (
         "google",

tests/agent/test_usage_pricing.py

@@ -415,3 +415,37 @@ def test_fireworks_rows_all_carry_cache_read_pricing():
         entry = _OFFICIAL_DOCS_PRICING[key]
         assert entry.cache_read_cost_per_million is not None, key
         assert entry.cache_read_cost_per_million < entry.input_cost_per_million, key
+
+
+def test_deepseek_v4_flash_pricing_entry_exists():
+    """Regression test: deepseek-v4-flash must have a pricing entry.
+
+    Before this fix, deepseek-v4-flash sessions showed $0.00 / cost_source
+    "none" because the _OFFICIAL_DOCS_PRICING table had an entry for
+    deepseek-v4-pro but not the (newer) flash model.  DeepSeek's /models
+    endpoint returns no pricing, so the official-docs snapshot is the only
+    source for direct-provider routes.
+    """
+    entry = get_pricing_entry(
+        "deepseek-v4-flash",
+        provider="deepseek",
+    )
+
+    assert entry is not None
+    assert float(entry.input_cost_per_million) == 0.14
+    assert float(entry.output_cost_per_million) == 0.28
+    assert float(entry.cache_read_cost_per_million) == 0.0028
+
+
+def test_deepseek_v4_flash_estimate_usage_cost():
+    """Ensure deepseek-v4-flash sessions get a dollar estimate, not $0/none."""
+    result = estimate_usage_cost(
+        "deepseek-v4-flash",
+        CanonicalUsage(input_tokens=1000000, output_tokens=500000),
+        provider="deepseek",
+    )
+
+    assert result.status == "estimated"
+    assert result.amount_usd is not None
+    # 1M input × $0.14/M + 500K output × $0.28/M = $0.14 + $0.14 = $0.28
+    assert float(result.amount_usd) == 0.28