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",