debug: add max_tokens trace logging to chat completions handler

Logs what max_tokens the client sends, whether gophergate injects
one from the registry, and the final value forwarded to the provider.
Helps trace output truncation issues.

cmd/test_lastused/main.go

@@ -0,0 +1,45 @@
+package main
+
+import (
+	"fmt"
+	"github.com/jmoiron/sqlx"
+	_ "modernc.org/sqlite"
+)
+
+type MyNullTime struct {
+	Time  interface{}
+	Type  string
+}
+
+func (n *MyNullTime) Scan(value interface{}) error {
+	n.Time = value
+	n.Type = fmt.Sprintf("%T", value)
+	return nil
+}
+
+func main() {
+	db, err := sqlx.Connect("sqlite", "/home/newkirk/Documents/projects/web_projects/gophergate/data/backups/llm_proxy.db.20260303T205057Z")
+	if err != nil {
+		fmt.Println("connect err:", err)
+		return
+	}
+	defer db.Close()
+
+	// Test 1: Direct column scan type
+	var d MyNullTime
+	db.Get(&d, "SELECT last_used_at FROM client_tokens WHERE client_id = ? LIMIT 1", "sk-opencode")
+	fmt.Printf("direct SELECT: GoType=%s value=%v\n", d.Type, d.Time)
+
+	// Test 2: MAX aggregate scan type
+	var m MyNullTime
+	db.Get(&m, "SELECT MAX(last_used_at) FROM client_tokens WHERE client_id = ?", "sk-opencode")
+	fmt.Printf("MAX SELECT:    GoType=%s value=%v\n", m.Type, m.Time)
+
+	// Test 3: peek at the raw driver types
+	row := db.QueryRow("SELECT last_used_at, MAX(last_used_at) FROM client_tokens WHERE client_id = ? LIMIT 1", "sk-opencode")
+	var a, b interface{}
+	row.Scan(&a, &b)
+	fmt.Printf("\nRaw Scan:\n")
+	fmt.Printf("  last_used_at:     type=%T val=%v\n", a, a)
+	fmt.Printf("  MAX(last_used_at): type=%T val=%v\n", b, b)
+}