mcp: use rand.Text and slog.DiscardHandler over internal implementation (#773)

Address TODOs in code requiring go 1.24 as minimum version

mcp/logging.go: use slog.DiscardHandler, over local implementation
mcp/util.go: use rand.Text, over local implementation

Fixes #772

examples/server/sequentialthinking/main.go

@@ -234,7 +234,7 @@ func deepCopyThoughts(thoughts []*Thought) []*Thought {
 func StartThinking(ctx context.Context, _ *mcp.CallToolRequest, args StartThinkingArgs) (*mcp.CallToolResult, any, error) {
 	sessionID := args.SessionID
 	if sessionID == "" {
-		sessionID = randText()
+		sessionID = rand.Text()
 	}
 
 	estimatedSteps := args.EstimatedSteps
@@ -480,20 +480,6 @@ func ThinkingHistory(ctx context.Context, req *mcp.ReadResourceRequest) (*mcp.Re
 	}, nil
 }
 
-// Copied from crypto/rand.
-// TODO: once 1.24 is assured, just use crypto/rand.
-const base32alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ234567"
-
-func randText() string {
-	// ⌈log₃₂ 2¹²⁸⌉ = 26 chars
-	src := make([]byte, 26)
-	rand.Read(src)
-	for i := range src {
-		src[i] = base32alphabet[src[i]%32]
-	}
-	return string(src)
-}
-
 func main() {
 	flag.Parse()