feat(M3-002): implement AnthropicAdapter for Claude Sonnet 4.6, Opus 4.6, and Haiku 4.5

Adds AnthropicAdapter implementing IProviderAdapter. Installs @anthropic-ai/sdk,
registers the three Claude models with the Pi ModelRegistry, implements healthCheck()
via client.models.list(), and createCompletion() with streaming via messages.stream().
Replaces the legacy inline registerAnthropicProvider() method in ProviderService.
Gracefully skips registration when ANTHROPIC_API_KEY is not set.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

pnpm-lock.yaml

@@ -41,6 +41,9 @@ importers:
 
   apps/gateway:
     dependencies:
+      '@anthropic-ai/sdk':
+        specifier: ^0.80.0
+        version: 0.80.0(zod@4.3.6)
       '@fastify/helmet':
         specifier: ^13.0.2
         version: 13.0.2
@@ -582,6 +585,15 @@ packages:
       zod:
         optional: true
 
+  '@anthropic-ai/sdk@0.80.0':
+    resolution: {integrity: sha512-WeXLn7zNVk3yjeshn+xZHvld6AoFUOR3Sep6pSoHho5YbSi6HwcirqgPA5ccFuW8QTVJAAU7N8uQQC6Wa9TG+g==}
+    hasBin: true
+    peerDependencies:
+      zod: ^3.25.0 || ^4.0.0
+    peerDependenciesMeta:
+      zod:
+        optional: true
+
   '@asamuzakjp/css-color@5.0.1':
     resolution: {integrity: sha512-2SZFvqMyvboVV1d15lMf7XiI3m7SDqXUuKaTymJYLN6dSGadqp+fVojqJlVoMlbZnlTmu3S0TLwLTJpvBMO1Aw==}
     engines: {node: ^20.19.0 || ^22.12.0 || >=24.0.0}
@@ -5937,6 +5949,12 @@ snapshots:
     optionalDependencies:
       zod: 4.3.6
 
+  '@anthropic-ai/sdk@0.80.0(zod@4.3.6)':
+    dependencies:
+      json-schema-to-ts: 3.1.1
+    optionalDependencies:
+      zod: 4.3.6
+
   '@asamuzakjp/css-color@5.0.1':
     dependencies:
       '@csstools/css-calc': 3.1.1(@csstools/css-parser-algorithms@4.0.0(@csstools/css-tokenizer@4.0.0))(@csstools/css-tokenizer@4.0.0)