fix(pricing): add deepseek-v4-pro to official docs pricing table

deepseek-v4-pro has been routable since v0.12 but was missing from
the _OFFICIAL_DOCS_PRICING table. Sessions using this model showed
as "unknown cost" in hermes insights instead of a dollar estimate.

Add pricing entry using published list prices:
- input: \$1.74/M tokens
- output: \$3.48/M tokens
- cache_read: \$0.0145/M tokens

Uses standard list rates (not the 75% promo) so estimates remain
accurate after promo expires 2026-05-31.

Closes #24218

tests/agent/test_usage_pricing.py

@@ -190,3 +190,37 @@ def test_custom_endpoint_models_api_pricing_is_supported(monkeypatch):
 
     assert float(entry.input_cost_per_million) == 0.5
     assert float(entry.output_cost_per_million) == 2.0
+
+
+def test_deepseek_v4_pro_pricing_entry_exists():
+    """Regression test: deepseek-v4-pro must have a pricing entry.
+
+    Before this fix, deepseek-v4-pro sessions showed as unknown cost
+    in hermes insights because the _OFFICIAL_DOCS_PRICING table had no
+    entry for that model.  See #24218.
+    """
+    entry = get_pricing_entry(
+        "deepseek-v4-pro",
+        provider="deepseek",
+    )
+
+    assert entry is not None
+    assert entry.input_cost_per_million is not None
+    assert entry.output_cost_per_million is not None
+    assert float(entry.input_cost_per_million) == 1.74
+    assert float(entry.output_cost_per_million) == 3.48
+    assert float(entry.cache_read_cost_per_million) == 0.0145
+
+
+def test_deepseek_v4_pro_estimate_usage_cost():
+    """Ensure deepseek-v4-pro sessions get a dollar estimate, not unknown."""
+    result = estimate_usage_cost(
+        "deepseek-v4-pro",
+        CanonicalUsage(input_tokens=1000000, output_tokens=500000),
+        provider="deepseek",
+    )
+
+    assert result.status == "estimated"
+    assert result.amount_usd is not None
+    # 1M input × $1.74/M + 500K output × $3.48/M = $1.74 + $1.74 = $3.48
+    assert float(result.amount_usd) == 3.48