fix(cost): add dated model ID for Claude Sonnet 4.6

PR #5579 added `claude-sonnet-4-6` but the Anthropic API returns
the dated version `claude-sonnet-4-6-20260217`. This caused $0 cost
calculations since the exact string match failed.

Adds the dated model ID to both the legacy cost lookup and unified
models mapping.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: colegottdank

packages/cost/providers/anthropic/index.ts

@@ -216,6 +216,20 @@ export const costs: ModelRow[] = [
       prompt_cache_creation_1h: 0.000006, // $6 / MTok
     },
   },
+  {
+    model: {
+      operator: "equals",
+      value: "claude-sonnet-4-6-20260217",
+    },
+    cost: {
+      prompt_token: 0.000003, // $3 / MTok
+      completion_token: 0.000015, // $15 / MTok
+      prompt_cache_write_token: 0.00000375, // 5m cache write: $3.75 / MTok
+      prompt_cache_read_token: 0.0000003, // Cache hits/refreshes: $0.30 / MTok
+      prompt_cache_creation_5m: 0.00000375, // $3.75 / MTok
+      prompt_cache_creation_1h: 0.000006, // $6 / MTok
+    },
+  },
   {
     model: {
       operator: "equals",

packages/cost/unified/models.ts

@@ -336,6 +336,10 @@ export const modelMapping: CreatorModelMapping = {
           provider: "ANTHROPIC",
           modelString: "claude-sonnet-4-6",
         },
+        {
+          provider: "ANTHROPIC",
+          modelString: "claude-sonnet-4-6-20260217",
+        },
         {
           provider: "BEDROCK",
           modelString: "anthropic.claude-sonnet-4-6-20260217-v1:0",