trace: add fuzz tests for TraceIDFromHex and SpanIDFromHex (#7577)

Co-authored-by: Robert Pająk <pellared@hotmail.com>

trace/trace_fuzz_test.go

@@ -0,0 +1,73 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package trace
+
+import (
+	"regexp"
+	"testing"
+)
+
+func FuzzTraceIDFromHex(f *testing.F) {
+	// Seed corpus with valid and edge-case examples.
+	f.Add("00000000000000000000000000000001") // Lowest valid (non-zero).
+	f.Add("0123456789abcdef0123456789abcdef")
+	f.Add("ffffffffffffffffffffffffffffffff") // Highest valid.
+	f.Add("0123456789abcdefabcdefabcdefabcd")
+	f.Add("invalidhexstringnot32chars") // Invalid.
+
+	validTraceIDRe := regexp.MustCompile(`^[0-9a-f]{32}$`)
+
+	f.Fuzz(func(t *testing.T, s string) {
+		id, err := TraceIDFromHex(s)
+
+		// OTel-valid TraceIDs: 32 lowercase hex chars, not all zeros.
+		validTraceID := validTraceIDRe.MatchString(s) && s != "00000000000000000000000000000000"
+		if validTraceID && err != nil {
+			t.Fatalf("expected no error for valid hex input: %q, got err: %v", s, err)
+		}
+		if !validTraceID && err == nil {
+			t.Fatalf("expected error for invalid input: %q", s)
+		}
+		if err != nil {
+			return
+		}
+
+		got := id.String()
+		if got != s {
+			t.Errorf("roundtrip mismatch: in=%q out=%q", s, got)
+		}
+	})
+}
+
+func FuzzSpanIDFromHex(f *testing.F) {
+	// Seed corpus with valid and edge-case examples.
+	f.Add("0000000000000001") // Lowest valid (non-zero).
+	f.Add("0123456789abcdef")
+	f.Add("ffffffffffffffff") // Highest valid.
+	f.Add("abcdefabcdefabcd")
+	f.Add("invalidhex") // Invalid.
+
+	validSpanIDRe := regexp.MustCompile(`^[0-9a-f]{16}$`)
+
+	f.Fuzz(func(t *testing.T, s string) {
+		id, err := SpanIDFromHex(s)
+
+		// OTel-valid SpanIDs: 16 lowercase hex chars, not all zeros.
+		validSpanID := validSpanIDRe.MatchString(s) && s != "0000000000000000"
+		if validSpanID && err != nil {
+			t.Fatalf("expected no error for valid hex input: %q, got err: %v", s, err)
+		}
+		if !validSpanID && err == nil {
+			t.Fatalf("expected error for invalid input: %q", s)
+		}
+		if err != nil {
+			return
+		}
+
+		got := id.String()
+		if got != s {
+			t.Errorf("roundtrip mismatch: in=%q out=%q", s, got)
+		}
+	})
+}