feat: add Anthropic sticky session digest chain matching via Trie

The previous fallback (step 3) in GenerateSessionHash hashed system +
all messages together, producing a different hash each round as the
conversation grew ([a] -> [a,b] -> [a,b,c]). This made fallback sticky
sessions ineffective for multi-turn conversations.

Implement per-message Trie digest chain matching (reusing Gemini's Trie
infrastructure) so that the previous round's chain is always a prefix
of the current round's chain, enabling reliable session affinity.

backend/internal/service/gateway_multiplatform_test.go

@@ -232,6 +232,14 @@ func (m *mockGatewayCacheForPlatform) SaveGeminiSession(ctx context.Context, gro
 	return nil
 }
 
+func (m *mockGatewayCacheForPlatform) FindAnthropicSession(ctx context.Context, groupID int64, prefixHash, digestChain string) (uuid string, accountID int64, found bool) {
+	return "", 0, false
+}
+
+func (m *mockGatewayCacheForPlatform) SaveAnthropicSession(ctx context.Context, groupID int64, prefixHash, digestChain, uuid string, accountID int64) error {
+	return nil
+}
+
 type mockGroupRepoForGateway struct {
 	groups           map[int64]*Group
 	getByIDCalls     int