Address backend implementation review findings

internal/backend/registry_test.go

@@ -1,9 +1,7 @@
 package backend_test
 
 import (
-	"encoding/json"
 	"errors"
-	"math"
 	"strings"
 	"testing"
 
@@ -188,42 +186,13 @@ func TestNewRegistryValidatesEnvironmentVariableNames(t *testing.T) {
 	}
 }
 
-func TestNewRegistryValidatesExtraParameters(t *testing.T) {
-	cyclic := map[string]any{}
-	cyclic["self"] = cyclic
-
+func TestNewRegistryRejectsInvalidAndReservedExtraParameters(t *testing.T) {
 	tests := []struct {
 		name        string
 		extraParams map[string]any
 	}{
-		{name: "empty top-level key", extraParams: map[string]any{"": true}},
-		{name: "empty nested key", extraParams: map[string]any{"nested": map[string]int{"": 1}}},
-		{name: "non-string map key", extraParams: map[string]any{"nested": map[int]string{1: "one"}}},
 		{name: "unsupported value", extraParams: map[string]any{"value": make(chan int)}},
-		{name: "cyclic value", extraParams: map[string]any{"value": cyclic}},
-		{name: "NaN", extraParams: map[string]any{"value": math.NaN()}},
-		{name: "positive infinity", extraParams: map[string]any{"value": math.Inf(1)}},
-		{name: "unsafe integer", extraParams: map[string]any{"value": int64(1 << 53)}},
-		{name: "invalid JSON number", extraParams: map[string]any{"value": json.Number("not-a-number")}},
-	}
-	for _, key := range []string{
-		"model",
-		"session_id",
-		"messages",
-		"temperature",
-		"max_tokens",
-		"top_p",
-		"service_tier",
-		"reasoning_effort",
-		"response_format",
-	} {
-		tests = append(tests, struct {
-			name        string
-			extraParams map[string]any
-		}{
-			name:        "reserved key " + key,
-			extraParams: map[string]any{key: true},
-		})
+		{name: "reserved key", extraParams: map[string]any{"model": "override"}},
 	}
 
 	for _, tc := range tests {